This is a variation on Rose Levy Beranbaum’s a classic chocolate butter cake:
2.25 ounces (1/3 cup) unsweetened cocoa (Dutch process)
8 ounces (1 cup) boiling water
3 large eggs (at room temperature)
2 1/4 teaspoons vanilla extract
8.25 ounces (1 3/4 cup + 2 tablespoons) sifted cake flour
10.5 ounces (1 1/2 cups) granulated sugar
1 tablespoon baking powder
3/4 teaspoon salt
6 ounces unsalted butter, softened
First, in a small bowl, combine the boiling water and cocoa and whisk to combine. Allow the mixture to cool to room temperature. Meanwhile, preheat your oven to 350 and prepare two 9″ layer pans according to the How to Prepare a Cake Pan for Baking post under the Techniques menu. Combine the dry ingredients in the bowl of a mixer fitted with a beater attachment and stir on low 30 seconds to combine. Crack the eggs into a small bowl.
When the cocoa mixture is cool, mix your batter. Add 1/4 of the cocoa mixture and the vanilla to the eggs and whisk lightly to combine. Set aside. Add the butter and remaining cocoa mixture to the dry ingredients and mix on low 30 seconds or so until all the ingredients are moistened. Scrape the bowl. Turn up the mixer to medium and beat 1 1/2 minutes until lighter in color and smooth. Add the egg mixture 1/3 at a time, beating 20 seconds between each addition and scraping the bowl down. Divide the batter into prepared pans and bake 25 to 35 minutes until the centers of the layers spring back lightly when touched. Let cool ten minutes then turn the layers out onto greased racks for 10 more minutes. Flip the layers right-side up to cool the rest of the way. Wrap airtight or freeze if you wish.