Preheat oven to 350F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Blend in eggs and vanilla.
Add flour; mix well. Refrigerate 1 hour, or until dough is easy to handle.
Shape dough into 1-inch balls; place, 2 inches apart, on greased baking sheets.
Bake 8 min. or just until set. (Do not overbake.)
Let stand on baking sheet 1 min.; transfer to wire racks. Cool completely.