Place chocolate in a medium bowl. Bring cream to boil in a small, heavy saucepan. Pour cream over chocolate. Let stand for 3 to 5 minutes; gently stir until smooth.
Add vanilla and stir to combine. Cover loosely with plastic wrap and refrigerate until firm, at least 3 hours.
Sift cocoa into a small bowl. For each truffle, use a measuring spoon to scoop up 1 teaspoon of chilled chocolate mixture; quickly roll into a 3/4 inch ball. Drop into cocoa; roll to coat. Chill truffles until firm. Store in an airtight container in refrigerator up to 2 weeks.