I have a yellow cake recipe that I really like, and I want to turn it into chocolate cake.

yay for successful recipe-tweaking!

Thanks everyone. I ended up swapping cocoa for flour one to one by weight. I meant to do 30 grams, but I poured a little fast and did closer to 40. I also added 1/4 tsp. baking soda and I swapped buttermilk for sour cream, since that's what I had in the fridge. It came out really good. I think I will take out the extra baking soda next time, since the cakes rose more than usual and came up past the edge of the pan.

i recommend getting a good chocolate cake recipe. this is not a small adjustment you're making to the recipe-- you are creating an entirely new one. you might not necessarily need to subtract flour to make room for the cocoa (especially since it's cake flour), but you might need to adjust the sugar to work with the cocoa (or not). if you are making 2-layers, 1/4 cup of cocoa does not sound like nearly enough. i think most recipes i make use at least 1/2 cup of cocoa powder, and the more you try to tweak the amounts, the more risk you run of having problems with the finished product.

that said, i'm all for experimentation and trying new things, and that there is buttermilk and baking soda in the recipe is a good start. tell us what happens!

P.S. In a perfect world, try and replacing are two separate words.

Tryreplacing 1/4 cup of flour with the same amount of cocoa. Add 1/4 teaspoon of baking soda and sift in with your other dry ingredients. Save that lovely bittersweet chocolate for your icing.