Beat pudding mix and 1 cup milk in medium bowl with whisk 2 min. Stir in COOL WHIP. Let stand 5 min. Meanwhile, cut cake horizontally into 2 layers with serrated knife.
Stack cake layers on plate, spreading pudding mixture between layers.
Microwave chocolate and butter in medium microwaveable bowl on HIGH 1 min. or until butter is melted; stir until chocolate is completely melted and mixture is well blended. Add sugar and remaining milk; mix well. Spread over cake. Refrigerate 1 hour.