Makes about 2 cups

One 12-ounce package bittersweet chocolate bits

¾ cup espresso or other strong coffee

1 pound unsalted butter, chilled and cut into pieces

2 teaspoons pure vanilla extract

Combine the chocolate and coffee in the top half of a double boiler over simmering water.  Heat, stirring constantly, for about 7 minutes or until the chocolate has melted and is incorporated into the coffee.

Remove from the heat but keep the double boiler intact.  Begin adding the butter, a piece at a time, beating to incorporate after each addition.  If the mixture gets too cold to incorporate the butter, return the double boiler to medium heat and again bring the water to a simmer.

When all of the butter has been added, stir in the vanilla.  Use immediately or pour into a clean container and store, covered and refrigerated, for up to 2 weeks.  Reheat when ready to serve.

This sauce, once firmed, can also be used to frost a cake.