Prepare cake batter as directed on package; pour into a greased 13" x 9" baking pan. Set aside. Pour milk and water into a large bowl; stir in dry puding mixes and sugar. Beat with a wire whisk for 2 minutes, or until well blended. Pour milk mixture into baking pan over cake batter; do not stir. Place baking pan on a baking sheet to catch any bubble-overs. Bake at 350 degrees for 55 minutes to one hour, until a toothpick inserted near the center comes out clean. Cool for 20 minutes to allow sauce to thicken. Dust with powdered sugar. Store leftoevers in the refrigerator.
To make a Halloween shape on the top of the cake, cut a simple pattern from parchment paper. Lay the pattern on top of the cake and dust with powdered sugar. Remove the paper pattern.