Mocha Chocolate Cake

Preheat the oven to 350 degrees. Butter or line with parchment paper 2 (8-inch) round cake pans. Sift the flour, sugar, cocoa, baking soda, baking powder, 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. Make sure the batter on the bottom and sides are thoroughly mixed. 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 them out onto a cooling rack and cool completely.

To do the frosting, place the chocolate in a heat-proof bowl set over a pan of simmering water. Stir until just melted and set aside until cooled to room temperature.

Using your electric mixer, 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. Dissolve 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! Make sure sure the frosting on bottom and sides of bowl are thoroughly mixed.

Place 1 layer, flat side up, on a flat plate or cake pedestal. Immediately spread frosting on the cooled cake. Starting with the top of the layer, cover with frosting. Place the second layer on top of that one, rounded side up, and spread the frosting evenly on the top and sides of the cake.