Make the sponge cake: Preheat the oven to 350°F. Grease and flour two 9-inch cake pans.
In the bowl of an electric mixer, whisk the eggs and sugar to combine. Place the mixing bowl over a medium pot of barely simmering water, and whisk until the sugar is dissolved and the mixture is pale and thick, 4 to 5 minutes.
Return the bowl to the mixer, fitted with the whip attachment. Whip on high speed until the mixture is very pale and thick; it should double in volume. Add the vanilla and mix to combine.
In a medium bowl, whisk the flour, baking powder, baking soda, and salt to combine.
Add the sour cream to the mixer and whisk until fully incorporated. Add the flour gradually, mixing just until combined and fully incorporated.
Divide the batter between the prepared pans and bake until a toothpick inserted into the center comes out clean, 40-45 minutes. Allow cakes to cool on a rack for 15 minutes before removing from pans and allowing to cool completely.
Slice the cakes into layers. Remove any domed part off the top of each cake—you’ll want all your layers to be nice and flat. Then, cut each cake into two layers. I use a serrated knife to slowly “score” the cake in an even line all the way around, then I follow the score marks to make the final cut.
Make the buttercream: In the bowl of an electric mixer fitted with the paddle attachment, beat room temperature butter until light and smooth, 1 to 2 minutes.
Gradually add the confectioners' sugar (to prevent it exploding upward when you turn on the mixer), and mix on medium-low speed to combine. Once all the sugar is added, raise speed to medium-high and cream until light and fluffy, 4 to 5 minutes.
Add vanilla extract or other flavoring and mix to combine. If using milk or cream, pour it in and mix just until incorporated.
Assemble the cake: Place the first layer on a cake stand and top with frosting (I like to use an ice cream scoop so I know how much frosting I'm using for each layer and it stays even). Be sure to fill the frosting all the way to the edge. To achieve this, frost all the way to the edge of the cake layer. When you place the next layer of cake on top, press down slightly—this stabilizes the layer and helps the frosting ooze out a bit more to the edge. Repeat with the remaining layers, ending with a cake layer on top.
Place the egg whites in the bowl of an electric mixer fitted with the whip attachment.
Combine the sugar and water in a medium pot and stir over medium heat until it comes to a boil. When it begins to boil, stop stirring and attach a candy thermometer to the pot (if any sugar crystals have washed up on the sides, brush them away using a pastry brush dipped in cool water). Continue to cook the syrup until it reaches 240° F (final desired temperature).
As soon as the sugar hits 230° F, begin whipping the egg whites on medium-high speed. The goal is to have the egg whites at soft peaks when the sugar reaches the 240° F.
With the mixer running, add the sugar syrup in a slow, steady stream. Continue to whip on high speed until the mixture reaches stiff peaks and the bowl is no longer noticeably warm to the touch. The meringue should be smooth and glossy, not clumpy or dry.
Beat in the vanilla and mix to combine.
Cover the entirety of your layer cake with the meringue, making as many peaks and swoops as possible.
With a culinary torch, toast the outside of the entire cake until the peaks and swoops become golden brown, or to desired toastiness.
I always carry three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's pie. My first cookbook, The Fearless Baker, is out on October 24, 2017.