Mix & bake the cake as instructed on the box ADDING 1 of the Crushed butterfinger bars into the mix before pouring batter in pan to cook in a 9 X 13 pan.
After cake is cooked & cooled, poke holes all over the top of the cake. Pour caramel ice cream topping on cake, then pour the sweetened condensed milk on top of that. Crush the 3 remaining butterfinger candy bars and sprinkle 1/2 of the crushed candy on top of the sweetened condensed milk layer that's on top of the cake. Finally "frost" cake with cool whip then take the remaining 1/2 of the crushed candy and sprinkle on the top of the cool whip topping. Keep refrigerated. YUMMMMM