Cut cake into 2 layers with serrated knife. Beat 1 cup milk and pudding mix with whisk 2 minutes. Stir in whipped topping. Let stand 5 min. Stack cake layers on serving plate, spreading pudding mixture between layers. Microwave chocolate and butter on high 1 minute. Stir until chocolate is melted. Add sugar and 2 Tbs. milk; mix well. Spread over top of cake only. Refrigerate 1 hour.
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