1. Melt the chocolates in a double boiler over low heat. Set aside to cool.
2. Place the egg yolks and sugar in a bowl. Beat on low speed with an electric mixer until combined. Increase the speed to high and continue mixing until the mixture is fluffy and lemon-colored, about 5 minutes.
3. Reduce the speed to low and add the chocolate. Mix until combined, about 1 minute. Add the coffee in a steady stream. Continue beating until incorporated. Add the butter a tablespoon at a time until incorporated. Increase the speed to high and whip until light and fluffy, about 5 minutes. Mix in the vanilla. Use the buttercream immediately or store covered in the refrigerator for up to 2 days. It may be frozen in a tightly covered container for up to three weeks.
Makes 4 cups (enough to frost a 9-inch, 2-layer cake). Per 1/4-cup: 310 calories, 21g carbohydrate, 3g protein, 27g fat, 100mg cholesterol.
Spread this buttercream atop this deliciousDevil's Food Cake.