Step 1

Microwave butter and 4 oz. chocolate in large microwaveable bowl on HIGH 1 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended.

Step 2

Add sugar; mix well. Blend in eggs and vanilla. Add flour; mix well.

Step 3

Refrigerate 1 hour.

Step 4

Heat oven to 350ºF. Shape dough into 64 (1-inch) balls; place, 2 inches apart, on baking sheets sprayed with cooking spray.

Step 5

Bake 8 min. or just until centers of cookies are set. (Do not overbake.) Cool on baking sheet 2 min. Press indentation into center of each cookie. Remove cookies to wire racks; cool completely.

Step 6

Microwave COOL WHIP and remaining chocolate in microwaveable bowl 1 min. or until chocolate is completely melted when stirred. Spoon into indentations in cookies, adding about 1/2 tsp. chocolate mixture to each. Top with marshmallows. Let stand until chocolate mixture is firm.