Makes 1 1/2 cups
Scald half-and-half and butter in a small heavy saucepan over medium heat. Remove from heat.
Place chocolate and vanilla in a medium heat-proof bowl. Add hot half-and-half and let sit for 2 minutes, then whisk until smooth. Serve slightly warm. (The sauce can be kept refrigerated in an airtight container for several days, but it must be returned to room temperature before serving.)