HEAT oven to 350°F.
MICROWAVE 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. Stir in flour until blended. Spread onto bottom of 13x9-inch pan sprayed with cooking spray.
BAKE 30 min. or until toothpick inserted in center comes out clean. Cool completely.
BEAT pudding mixes and milk in large bowl with whisk 2 min. (Pudding will be thick.) Stir in half the COOL WHIP. Spoon over brownie; top with remaining COOL WHIP. Refrigerate 2 hours.