Melt in a double boiler (or a heatproof bowl set over a pan of simmering water):
- 3 ounces unsweetened chocolate, coarsely chopped
- 3 tablespoons unsalted butter
Remove from the heat and stir in:
- ¼ cup hot coffee, cream, or milk
- 1 teaspoon vanilla
Gradually add, beating until spreadable:
- 2 cups powdered sugar, or to taste, sifted