School-Party Sheet Cake

September 1, 2016

Test Kitchen-Approved

Author Notes: When I bring treats to my girls’ school parties, I don’t want them to be mortified by their mom’s weird food—nor do I want to turn their classmates into hyperactive rascals with empty calories. That means something that looks familiar but tastes better than the original. Enter this easy sheet cake. Zucchini keeps the cocoa–chocolate chip cake extra moist, and sweet potato swirls into a frosting as creamy as that canned stuff, with far more flavor (and no added sugar). Dark chocolate—semisweet or a lower-percentage bittersweet—makes the cake luxurious and sophisticated enough for adults. Kids may ask about the bits of green zucchini in the cake. I’m not a believer in sneaking in veggies, so I tell them. Even if they balk, they still gobble up the cake once they take a bite or hear their friends raving. Party on!

–Use unsweetened pure canned sweet potato puree; it’s usually stocked near the canned pumpkin. Freshly cooked and pureed sweet potato is too granular and thick.

-Chocolate with a cacao content between 55 and 60% makes the frosting perfectly sweet and smooth.

-A glass or ceramic dish is nice for a sheet cake. Bake the cake in the greased dish, cool completely, and frost. You can cut it into pieces right in the dish. I have a Pyrex dish that comes with a stiff plastic lid, making it simple to tote to parties at school or a friend’s house.

-If you plan to cut your cake ahead and place the pieces on a serving platter, use a metal pan. Generally I prefer straight-sided metal cake pans, which produce clean edges. Line the bottom and sides with foil or parchment paper and grease the foil or paper. Once the cake has cooled completely, lift it out using the foil, then frost and cut it. To get perfect slices, you can freeze the frosted cake until firm before slicing, then bring it to room temperature before serving.

Excerpted from BETTER BAKING, (c) 2016 by Genevieve Ko. Reproduced by permission of Houghton Mifflin Harcourt/Rux Martin Books. All rights reserved.
Genevieve Ko

Makes: one 9-by-13 inch cake


For the chocolate-zucchini cake:

  • 1 pound (453g) zucchini (about 4 small), trimmed
  • 2 1/2 cups (360g) white whole wheat flour
  • 1/2 cup (48g) unsweetened cocoa powder
  • 1 teaspoon ground cinnamon
  • 1 teaspoon baking soda
  • 1/2 teaspoon baking powder
  • 1/2 teaspoon salt
  • 1 1/2 cups (312g) sugar
  • 1 cup (245g) buttermilk, at room temperature
  • 1/2 cup (112g) grapeseed or other neutral oil
  • 3 large eggs, at room temperature
  • 1 teaspoon pure vanilla extract
  • 1 cup (180g) semisweet chocolate chips

For the sweet potato frosting:

  • 1 (15-ounce; 425-g) can pure sweet potato puree
  • 10 ounces (283 g) semisweet or bittersweet chocolate, finely chopped (1⅔ cups)
In This Recipe


  1. Position a rack in the center of the oven and preheat to 325°F. Coat a 9-by-13-by 2-inch cake pan or dish with nonstick cooking spray. If you’re using a metal pan, line the bottom and sides with foil or parchment paper and spray again.
  2. Set a box grater on some paper towels and grate the zucchini on the large holes. Spread it out on the paper towels, top with more paper towels, and press gently to remove excess moisture.
  3. Sift the flour, cocoa powder, cinnamon, baking soda, baking powder, and salt into a large bowl. Whisk the sugar, buttermilk, oil, eggs, and vanilla in a medium bowl until very smooth. Make a well in the dry ingredients and pour in the wet ingredients. Whisk, gradually drawing in the dry ingredients, just until smooth. Fold in the zucchini and chocolate chips with a silicone spatula until evenly incorporated. Spread the batter in an even layer in the prepared pan.
  4. Bake the cake until a toothpick inserted in the center comes out clean and the top springs back a little when lightly pressed with a fingertip, 45 to 50 minutes. Cool completely in the pan on a rack.
  5. Meanwhile, make the frosting: Bring the sweet potato puree to a simmer in a large saucepan over medium heat, stirring frequently. Remove from the heat and add the chocolate. Stir until smooth. Cool, stirring occasionally, until the mixture is at room temperature and the consistency of canned frosting. It should hold soft peaks when you lift the spatula from the pan but not be stiff. Spread the frosting all over the top of the cake, creating swoops and swirls.
  6. The cake is best the day it’s made, when the chocolate chips are still a bit melty and the frosting is soft, but it will keep in the refrigerator for up to 3 days.

More Great Recipes:
Cake|Buttermilk|Chocolate|Vegetable|Serves a Crowd|Fall|Summer|Winter|Dessert

Reviews (32) Questions (1)

32 Reviews

RHo February 12, 2018
How is this a sheet cake?<br />Is it not possible to make your own sweet potato purée similar to the canned one?
Julie February 8, 2018
Also, can the sugar be swapped out for either coconut sugar or honey?
Julie February 8, 2018
Genevieve, Can this cake be made "paleo" by swapping out the wheat flour for almond and coconut flour (or a blend with tapioca starch)? If so, what would the amounts of each be?
AntoniaJames February 2, 2018
What a great idea! Must try. ;o)
Elana November 30, 2017
Could you use canned pumpkin puree instead of sweet potato? Canned sweet potato is not available where I live. Thanks!
Erika L. April 11, 2017
Has anyone used this recipe for cupcakes?
senorahughes March 2, 2017
Hi there, has anyone tried using this frosting to fill a layer cake? I'm making an 8'' 4 layer birthday cake, but want to make the frosting as sugar free as possible and this sounds perfect. Don't want a disaster though! :) Thank you!
Rob L. May 8, 2017
I made this yesterday as a layer cake with two 9-inch pans and it turned out great! I thought there wouldn't be enough frosting so I made about 1.5x the amount and ended up having too much. The original recipe would probably have been enough to cover it and 1.25x would be enough to generously cover it.<br /><br />A couple other thoughts... I wasn't sure how I'd feel about the cinnamon, but now I'm sold. You can definitely detect both the zucchini and sweet potato as sort of flavor undertones and the cinnamon helps smooth all of that over. And Genevieve mentions in the recipe that the cake is best on the day it's made and I totally agree. By the next day it was definitely more dry than other types of cake. But on the day of it was fantastic!
Amanda B. December 10, 2016
Alrighty, this cake is the BOMB. Everything about it was perfect. Used bittersweet chocolate in the chips and frosting and it was still plenty sweet. And the frosting, OH the frosting. It did not disappoint. Thanks, Genevieve, it made my sister a very happy birthday girl. (oh yes, it also came out looking just like the lovely picture :-)
Author Comment
Genevieve K. December 11, 2016
Amanda! I'm so glad you and your sister enjoyed the cake so much! So many of Better Baking's recipes were created for my lovely sister...nothing like baking for family. Happy baking!
sing December 2, 2016
Is the sweet potato frosting sticky enough to use for building ginger bread house??
Author Comment
Genevieve K. December 11, 2016
So sorry I just saw this!!! You've probably figured by now that it isn't stiff enough to build a gingerbread house, but it'd make for some pretty great mud on the "ground" under the house! Happy baking!
PssP November 5, 2016
Hi! Truly mind blown by the sweet-potato puree in the frosting!! Awesome idea! If i make a puree of steamed sweet potatoes, how many "cups" of the sweet potato puree would i need? Also, for the cake.... wanted to know if i can substitute regular APFlour for the wheat flour?
Author Comment
Genevieve K. November 5, 2016
Hi PssP, You would need 1 3/4 cups sweet potato puree. If the potatoes are really watery after steaming, you may want to drain the puree through a fine-sieve to get rid of excess water first. And you can substitute regular AP flour in equal amounts for the wheat flour. Happy baking! Genevieve
Linda October 31, 2016
Can the frosting be made with canned sweet potatoes - I can't find kosher sweet potato puree
Author Comment
Genevieve K. November 1, 2016
Hi Linda, Yes! Drain them really well, then puree them in a blender or food processor until really smooth. Happy baking! Genevieve
CBee October 31, 2016
Thanks for sharing this — it looks delicious! Would reducing the 1.5 cups of sugar in the cake seriously (read: negatively) affect its flavor? I know this recipe benefits sugar-wise (if you will) from not having it added in the frosting, but I'm curious if one could afford to reduce the sugar in the cake itself.
Author Comment
Genevieve K. November 1, 2016
Hi CBee, I actually don't like my sweets too sweet, so everything I created for Better Baking (the book from which this recipes comes) has the least amount of sugar possible before the results turn out less than delicious. You can leave out the chocolate chips though to reduce the total sugar and that won't affect the cake. You can halve the frosting recipe as well. It makes enough for a lovely layer, but you could enjoy the cake with a thinner shmear. Last but not least, this cake is quite big--13x9 and nearly 2 inches tall--so you can cut the pieces smaller for a little less sugar intake. Hope that helps! Genevieve
CBee November 2, 2016
Thank you, Genevieve! This helps a great deal. I figured you were using the least amount of sugar possible, but I figured I would double check. I'll definitely consider your suggestions.
Karen P. October 29, 2016
Intriguing! Could Food52 post nutritional info for the frosting, esp compared to traditional chocolate frosting.
Beth H. October 27, 2016
I want to make this came but I need it to be gluten free. Any suggestions? Thank you.<br />
Beth H. October 27, 2016
"cake" not came
Author Comment
Genevieve K. October 29, 2016
Hi Beth, There are a bunch of yummy gluten-free options in Better Baking, the book for which I created this recipe. For something similar to a sheet cake, you could slather this on the Buckwheat-Cocoa Banana Bread Bars (page 78). You could also spread or pipe it on Flourless Chocolate Hazelnut Cupcakes (page 267). If you want a death-by-chocolate sorta thing, you could spread it on Fudgy Flourless Brownies (page 189) or Chocolate Mousse Cake (255). Happy baking! Genevieve
Danielle October 27, 2016
Thoughts on an egg substitute for the cake? I do flax eggs sometimes, or applesauce, or the liquid from a can of chick peas. But every recipe is slightly different so I wondered if you had a suggestion for this one! Thanks!
Author Comment
Genevieve K. October 29, 2016
Hi Danielle, If you have you copy of Better Baking, you can use this frosting on the Purely Pumpkin Muffins (page 44), which are fluffy and sweet enough to be cupcakes. (But, you know, they'd seem fine at breakfast too!) You can put it in a piping bag if you want to pipe swirls on top! Happy baking! Genevieve
Pat E. October 26, 2016
Has anyone made this with homemade sweet potato purée? Should be pretty easy to make
Author Comment
Genevieve K. October 26, 2016
Hi Pat,<br />I tested this recipe quite a few times when developing it for Better Baking with homemade sweet potato puree. I ended up calling for canned sweet potatoes to get consistent results in terms of taste and texture. To get a super smooth, silky frosting with homemade puree, you have to blend steamed sweet potatoes really well and then press them through a very fine-mesh chinois or sieve. Also, be sure to use really fresh sweet potatoes as they're sweeter and less starchy. With good sweet potatoes (and good chocolate!), you'll end up with an extra delicious frosting! Enjoy!!!
Author Comment
Genevieve K. October 26, 2016
Hi Ann! I tried this recipe with canned pumpkin and it's too pasty and earthy-tasting for a really luscious frosting. My local store keeps the canned sweet potato pretty well hidden, but the cans should be more readily available with Thanksgiving coming. Just be sure to buy the kind with no sugar added. This is the one I like:<br />You can do this with pureed sweet potato baby food too. I know that sounds weird, but it works! Baby food tends to be thinner than the canned stuff, so the frosting will take longer to set up and not be quite as thick. Hope that helps!
Ann L. October 26, 2016
Do you think canned pumpkin would work in place of the sweet potato? I just haven't seen canned sweet potato in grocery stores where I live. Thank you.
BerryBaby October 21, 2016
Thank looks delicious! More than 25 years ago the local school banned bringing in baked treats, only store bought and wrapped are allowed. How I wish I could made treats. This would have been a big hit!
Matilda L. October 20, 2016
The sweet-potato puree frosting blew my mind a little. I have to try this!