1/2pound2 sticks unsalted butter, at room temperature
1extra-large egg yolkat room temperature
1teaspoonpure vanilla extract
1 1/4cupssifted confectioners' sugar
1tablespooninstant coffee powderdissolved in 2 teaspoons of hot tap water
Preheat the oven to 350 degrees F. Butter two 8-inch x 2-inch round cake pans. Line with parchment paper, then butter and flour the pans.
In the bowl of an electric mixer, sift the flour, sugar, cocoa, baking soda, baking powder, and salt. Using the paddle attachment, mix on low speed until combined.
In a medium bowl, add the buttermilk, oil, eggs, and vanilla and mix well using a whisk.
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 30 to 40 minutes depending on your oven (mine took 30 minutes), until a toothpick comes out clean when inserted in the middle of the cake.
Leave the cakes in the pans for 30 minutes to cool, then turn them out onto a cooling rack and cool completely.
Place 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.
Place the chocolate 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.
In 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.
On low speed, add the chocolate and dissolved coffee to the butter mixture and mix until blended.
The batter will be more liquidy when mixing and this is totally normal. It will firm up as it bakes.