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 beat for 3 more minutes. Turn the mixer to low, slowly add the confectioners’ sugar, then beat at medium speed, scraping down the bowl as needed, until smooth and creamy. Dissolve the coffee powder in 2 teaspoons of hot tap water. On low speed, add the chocolate and coffee to the butter mixture and mix until blended. Don’t whip! Spread onto the cooled cake. (Don’t wait too long, this ‘stiffens up’ when cooled completely!)