Cut 1 round yellow cake layer into 2 layers with serrated knife. Beat 1 cup milk and 1 pkg (3.4 oz) JELL-O Vanilla Flavor Instant Pudding with a whisk for 2 minutes. Stir in 1 1/2 cups COOL WHIP Whipped Topping. Let stand for 5 minutes.
Stack cake layers on serving plate, spreading pudding mixture between layers.
Microwave 1 square BAKER'S Unsweetened Chocolate and 1 tbsp butter on HIGH for 1 minute. Stir until chocolate is melted. Add 3/4 cup powdered sugar and 2 tbsp milk. Mix well. Spread over cake.
Refrigerate 1 hour. Refrigerate leftovers.