Thaw pastry sheet at room temperature for 30 minutes. Heat oven to 400 degrees. Microwave chocolate and butter in microwavable bowl on high for 1 minute or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Mix in eggs and vanilla. Stir in flour.

Unfold each pastry sheet on lightly floured surface. Roll into 15x12-inch rectangle. Cut into 20 (3-inch) squares. Press 1 square into bottoms of 3-inch muffin cups. Spoon about 1 tablespoon chocolate mixture into center of each pastry square. Bake 15 to 20 minutes or until pastry is golden. Remove from muffin pans. Cool on ware racks. Sprinkle with powdered sugar, if desired.