Prepare and bake cake mix as directed on package for 13 X 9-inch baking pan. Remove from oven. Immediately poke holes down through cake to pan at 1-inch intervals with round handle of or poke holes with a plastic drinking straw, using turning motion to make large holes.
Pour milk into large bowl. Add pudding mix. Beat with wire whisk for 2 minutes. Quickly pour about 1/2 of the thin pudding mixture evenly over warm cake and into holes. Let remaining pudding mixture stand to thicken slightly. Spoon over top of cake, swirling to frost cake.
Refrigerate at least 1 hour or until read to serve.