Bake cake in a 9x13-inch pan according to direction on the package.
While cake is baking, mix milk and caramel topping until well blended. When the cake is done and while it's still hot, poke holes in it with a fork or straw. Pour milk mixture over the cake. Allow cake to cool completely.
Sprinkle two candy bars over cake. Spread Cool Whip over the top, then sprinkle the remaining candy bars on top. Chill.
Tip: I like to freeze the butterfingers and crush them right in the wrapper.