MICROWAVE 2 cups of the marshmallows and the milk in large microwaveable bowl on HIGH 1-1/2 min. or until marshmallows are completely melted, stirring after 1 min. Stir until mixture is well blended. Refrigerate 15 min. or until completely cooled.
MEANWHILE, reserve 8 of the wafers for later use. Finely crush remaining 32 wafers; mix with the butter. Press firmly onto bottom of 9-inch square pan; set aside. Microwave chocolate in medium microwaveable bowl on HIGH 1 to 1-1/2 min. or until chocolate is almost melted; stir with wire whisk until completely melted. Stir in 1 cup of the whipped topping; spread over crust. Gently stir remaining 1 cup whipped topping into the cooled marshmallow mixture; spread over chocolate layer in crust.
REFRIGERATE 3 hours or until firm. Cut remaining 8 wafers in half. Add to top of dessert along with the remaining 1/2 cup marshmallows just before serving. Cut into 16 squares. Store leftover dessert in refrigerator.