Prepare cake mix according to package directions, using two greased 9-in. round baking pans. Cool completely.
In a large bowl, whisk milk and pudding mixes for 2 minutes. Let stand for 2 minutes or until soft-set. Spread evenly between cake layers and stack on serving plate. Frost top and sides of cake with whipped topping. Serve immediately or cover and refrigerate until ready to serve. Store leftovers in refrigerator.