HEAT oven to 350ºF.
PREPARE cake batter as directed on package; pour into 13×9-inch baking dish sprayed with cooking spray.
BEAT dry pudding mixes, granulated sugar, milk and water with whisk 2 min.; pour over batter in dish. Place baking dish on baking sheet. (Baking sheet will catch any sauce that might bubble over sides of dish as dessert bakes.)
BAKE 55 min. to 1 hour or until toothpick inserted in center comes out clean. Cool 20 min. (Sauce will thicken slightly as it cools.) Sprinkle with powdered sugar. Serve warm. Refrigerate leftovers.