- 6 ounces good semisweet chocolate (Ina recommends Callebaut.)
- 1/2 pound (2 sticks) unsalted butter, at room temperature
- 1 extra-large egg yolk, at room temperature
- 1 teaspoon pure vanilla extract
- 1 1/4 cups (5 3/8 oz | 152 g) sifted confectioners’ sugar
- 1 tablespoon instant coffee powder (optional — I skipped this)
- 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. Alternatively: Melt in the microwave at 30 second to 1 minute intervals until spreadable.
- 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 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. If you are adding the coffee, dissolve the 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.