PREPARE cake batter and bake as directed on package for 13x9-inch baking pan. Cool cake in pan 15 min. Pierce cake with large fork at 1/2-inch intervals.
STIR boiling water into dry gelatin mix in small bowl 2 min. until completely dissolved. Stir in cold water; carefully pour over cake. Refrigerate 3 hours.
FROST cake with whipped topping. Refrigerate at least 1 hour before serving. Store leftovers in refrigerator.