1. Prepare cake batter. Bake in 13x9x2-inch baking pan according to package directions. Remove from oven; do not remove cake from pan.
2. Using handle of wooden spoon, immediately poke holes through top of cake to bottom of pan, spacing holes about 1 inch apart, covering length and width of cake. Immediately spread or pour fudge topping evenly over cake, making sure entire top is covered and mixture has flowed into holes.
3. Cool completely on wire rack; spread whipped topping evenly over top of cake. Cover; refrigerate several hours or overnight until well chilled. Garnish with sprinkles. Cover; refrigerate leftover cake. 12 to 15 servings.
* HOT FUDGE VARIATION: Bake and poke holes in cake as directed above. Place open jar HERSHEY'S Chocolate Shoppe Fudge Topping (Hot Fudge flavors) in microwave oven; heat as directed on label. Carefully spread fudge topping on cake; proceed as directed above.