0.5 lbunsalted butter (2 sticks), at room temperature
1extra-large egg yolk, at room temperature
1 tsppure vanilla extract
1.25 cupsifted confectioners' sugar
1 tbspinstant coffee powder
Method
1Preheat oven to 350°F. Butter two 8×2-inch round cake pans. Line with parchment paper, then butter and flour the pans.
2Sift the flour, sugar, cocoa, baking soda, and salt into the bowl of an electric mixer fitted with a paddle attachment and mix on low speed until combined. In another bowl, combine the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry. With mixer still on low, add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula. Pour the batter into the prepared pans and bake for 35 to 40 minutes, until a cake tester comes out clean. Cool in the pans for 30 minutes, then turn out onto a cooling rack and cool completely.
3Place 1 layer, flat side up, on a flat plate or cake pedestal. With a knife or offset spatula, spread the top with frosting. Place the second layer on top, rounded side up, and spread the frosting evenly on the top and sides of the cake.
4CHOCOLATE FROSTING — Chop the chocolate and place it in a heat-proof bowl set over a pan of simmering water. Stir until just melted and set aside until cooled to room temperature.
5In the bowl of an electric mixer fitted with a paddle attachment, beat the butter on medium-high speed until light yellow and fluffy, about 3 minutes. Add the egg yolk and vanilla and continue beating for 3 minutes. Turn the mixer to low, gradually add the confectioners' sugar, then beat at medium speed, scraping down the bowl as necessary, until smooth and creamy.
6Dissolve the coffee powder in 2 teaspoons of the hottest tap water. On low speed, add the chocolate and coffee to the butter mixture and mix until blended. Don't whip! Spread immediately on the cooled cake.
Notes
Frosting must be spread immediately after mixing — it sets quickly. The coffee in both the cake batter and frosting deepens the chocolate flavor without tasting distinctly of coffee.