Mix cake according to package directions, Bake in 13x9 pan as directed. Remove from oven. Poke holes quickly down through cake with round handle of wooden spoon. Only after poking holes, Combine pudding with sugar in large bowl. Gradually stir in milk. Beat at low speed of electric mixture not more than 1 minute. Quickly before pudding thickens pour 1/2 of pudding evenly over warm cake and into holes to make stripe. Allow rest of pudding to thicken and spoon over top, swirling to frost cake. Chill at least 1 hour.