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
confectionersí 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. Donít whip! Spread immediately on the cooled cake.
Note- I use Valrhona Le Noir 56 percent Cacao Semisweet
Chocolate. You can use also use a good bittersweet chocolate but
donít use chocolate chips because they have stabilizers in them.