1/2pound2 sticks unsalted butter, at room temperature
1extra-large egg yolkat room temperature
1teaspoonpure vanilla extract
1 1/4cupssifted confectioners’ sugar
1/2tablespooninstant coffee powdercut by 50% from original recipe
Instructions
Cake
Preheat the oven to 350 degrees.
Butter two 8-inch x 2-inch round cake pans. Line with parchment paper.
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, mix 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 cake pans and bake for 35 to 40 minutes, or until a toothpick comes out clean.
Cool in the pans for 30 minutes, then turn them out onto a cooling rack and cool completely.
Mocha Buttercream Frosting
Chop the chocolate and place in a double boiler to melt. Stir until just melted and set aside to cool to room temperature.
Using 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 adding the confectioners’ sugar, then beat at medium speed until smooth and creamy.
Dissolve the coffee powder in 1 teaspoon 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.