½ c. Dutch processed cocoa, divided (Dutch processed cocoa is a lovely dark brown color, and reacts differently than standard Hershey's). You can buy it in most stores, or order from Penzey's.
4 oz. semisweet chocolate (chocolate chips will work in a pinch)
2 c. sugar, divided
½ c. hot water
4 large eggs
2 large egg yolks
2 tsp vanilla extract
12 T salted butter (1½ sticks), very soft
1½ cups + 2 T unbleached all-purpose flour (8¾ ounces)
1½ teaspoons baking soda
1 tsp table salt
1 cup buttermilk
Baker's Joy, or extra butter and flour for preparing pans
For assembly:
1 8 oz. jar tart jam - apricot, raspberry or plum preferred
White chocolate cream cheese frosting
Instructions
Preheat oven to 350 degrees, prepare 2 9" cake pans by spraying with baker's joy or buttering and then flouring.
Combine chocolate, ¼ c. cocoa powder, and hot water in a heatproof bowl, set it over ; set bowl over saucepan containing 1 inch of simmering water and stir until the chocolate is melted. Add ¾ cup sugar to chocolate mixture and stir until thick and glossy, 1 to 2 minutes. Remove bowl from heat and set aside to cool.
In a standing mixer with a balloon whisk attachment, whisk the eggs and yolks until combined, then add remaining 1¼ c. sugar, increase the speed to high, and beat until fluffy and pale yellow. Add the cooled chocolate mixture and vanilla and mix until thoroughly incorporated, then, while mixer is running, add soft butter 1 T at a time and mix until thoroughly incorporated.
Combine the flour, soda, remaining ¼ c. cocoa and salt. Add about ⅓ of the flour mixture to the mixer, followed by half of the buttermilk mixture, mixing until combined after each addition, and repeat until flour and buttermilk are thoroughly incorporated. Divide batter evenly between prepared cake pans; smooth batter to edges of pan with spatula.
Bake cakes until a skewer inserted into center comes out with a few crumbs attached, 25 to 30 minutes. Cool cakes in pans 15 minutes, then invert onto wire rack. Cool cakes to room temperature before frosting, 45 to 60 minutes, or make the cakes a day or two ahead and freeze the layers, wrapped in plastic wrap and aluminum foil, then frost the frozen layers.
To assemble the cake:
Place dab of frosting on cake platter to hold cake in place, Place bottom layer of cake on center of platter. Spread a thin layer of frosting on top of cake, followed by jam. Center 2nd layer of cake atop the first. Spread a thin layer of frosting on the cake, starting with the sides and then moving to the top. An angled spatula is your friend here, and a turntable is even better. Get the frosting as smooth as possible, but it's OK if it's not entirely opaque. Chill until firm. When the crumb coat has chilled, coat the remainder of the cake in frosting, starting with the sides and then smoothing the top. Get as smooth as possible, then chill until serving.
Optional: decorate with chocolate plastic cut outs.
Notes
Adapted from Cook's Illustrated, March, 2006
Recipe by The Domestic Front at http://www.thedomesticfront.com/chocolate-birthday-cake/