Chop chocolate, and place in a bowl. Bring cream to a boil in a small, heavy saucepan; pour over chocolate. Let stand for 5 minutes; stir until smooth. Stir in liqueur. Refrigerate until firm, at least 3 hours or overnight.
Scoop up 1/2 teaspoon of chilled chocolate, and quickly roll into a ball. Continue until mixture is used up. Roll each truffle in cocoa to coat. Chill until firm.
Store in an airtight container in the refrigerator for up to 2 weeks.