Dietary guidance: v
6 ounces bittersweet chocolate (e.g., Callebaut)
1/2 pound (2 sticks) unsalted butter, at room temperature
1 extra-large egg yolk
1 teaspoon pure vanilla
1 1/4 cups confectioners' sugar
2 teaspoons coffee (decaf ok)
1. Chop chocolate into small pieces and melt gently in a double boiler until chocolate is very soft. Remove from heat and stir until all the pieces are melted. Let cool until almost room temperature.
2. In the bowl of a electric mixer, with paddle attachment, beat butter until light and fluffy, about 3 minutes. Add egg yolk and vanilla and mix well, beating about 3 minutes. On low speed, gradually add confectioners' sugar. Continuing on low speed, add chocolate and mix in slightly, then add coffee. Mix until blended, but do not whip. Spread on cake.