PREHEAT oven to 400°F. Microwave chocolate and butter in large microwavable bowl on HIGH 1 minute or until butter is melted. Stir until chocolate is completely melted. Add sugar; mix well. Blend in eggs and vanilla. Stir in flour until well blended.
UNFOLD pastry sheet on lightly floured surface. Roll into 15x12-inch rectangle. Cut into 20 (3-inch) squares. Press 1 square onto bottom and up side of each of 20 medium muffin cups. Fill each with 1 Tbsp. of the chocolate mixture.
BAKE 15 to 20 minutes or until pastry is golden brown. Remove from muffin pans. Cool on wire racks. Sprinkle with powdered sugar, if desired.