6 oz good semi sweet chocolate (I used a Ghirardelli baking bar)
2 sticks unsalted butter
1 extra large egg yolk, room temp
1 teaspoon pure vanilla extract
1 1/4 cups sifted confectioners sugar
1 tablespoon instant coffee
Chop chocolate and place in heat-proof bowl set over a pan of simmering water. Stir until just melted and set aside until cooled to room temp.
In the bowl of eclectic 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, 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.