6 ounces good semisweet chocolate (recommended: Callebaut)
1/2 pound (2 sticks) unsalted butter, at room temperature
1 extra-large egg yolk, at room temperature
1 teaspoon pure vanilla extract
1 1/4 cups sifted confectioners’ sugar
1 tablespoon instant coffee powder
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.
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.
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! Spread immediately on the cooled cake.