Chop the chocolate and place it in a heatproof bowl over a pan over simmering water. Stir until just melted and set aside until cooled to room temperature. Beat the butter in the bowl of an electric mixer fitted with the paddle attachment 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 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. Note: I use Valrhona Le Noir 56% Cacao Semisweet Chocolate. You can use also use a good bittersweet chocolate but don’t use chocolate chips because they have stabilizers in them.