Microwave unsweetened chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted.
Add sugar; mix well. Blend in eggs and vanilla.
Add flour; mix well. Refrigerate 1 hour or until dough is easy to handle.
Heat oven to 350F. Shape dough into 1-inch balls; place, 2 inches apart, on baking sheets sprayed with cooking spray.
Bake 8 min. or just until set. (Do not overbake.) Cool on baking sheet 1 min.; remove to wire racks. Cool completely.
Microwave COOL WHIP and semi-sweet chocolate in microwaveable bowl on HIGH 1-1/2 min. or until chocolate is completely melted and mixture is well blended, stirring after 1 min.