HEAT oven to 350°F. Prepare cake batter as directed on package. Pour into greased 13x9-inch baking dish; set aside.

POUR milk and water into large bowl. Add pudding mixes and sugar. Beat with whisk 2 min. or until blended. Pour over batter.

PLACE baking dish on baking sheet to catch any sauce that bubbles over sides of dish as dessert bakes. Bake 55 min. to 1 hour or until toothpick inserted in center of cake layer comes out clean. Cool 20 min. (Sauce will thicken slightly as it cools). Spoon into serving dishes; top with COOL WHIP.