Prepare and bake cake according to package directions, using a greased 13x9-in. baking pan. Cool on a wire rack.
Using the handle of a wooden spoon, poke holes in cake. Pour 3/4 cup caramel topping into holes. Spoon remaining caramel over cake. Top with whipped topping. Sprinkle with candy. Refrigerate for at least 2 hours before serving.