PREPARE cake batter and bake as directed on package for 13x9-inch pan. Cool cake in pan 15 min. Pierce cake with large fork at 1/2-inch intervals.
ADD boiling water to gelatin mix in small bowl; stir 2 min. until completely dissolved. Stir in cold water; pour over cake. Refrigerate 3 hours.
FROST cake with COOL WHIP. Refrigerate 1 hour.