Bake a white cake in a 13x9 pan as directed on the box. Let cake cool completely. Prick with a fork at 1/2 inch intervals. Dissolve 2 (4 serving size) pkgs Jell-O mix into 2 cups boiling water. Carefully spoon dissolved Jell-O over the cake. Chill cake for 3 to 4 hrs. Top with 3 1/2 cup Cool Whip, thawed.