In a large mixing bowl, beat cake mix, water, and egg whites until blended. Pour into 9 x 13 baking pan coated with nonstick spray. Bake at 350 for 35-40 minutes or until toothpick comes out clean. Cool on wire rack.
With a meat fork or wooden skewer, poke holes about 2 inches apart into cake. Slowly pour condensed milk and caramel over cake, sprinkle with 2/3 candy bars. Spread with whipped topping, sprinkle with remaining candy bars. Refrigerate until serving.