Preheat oven to 350 degrees F. Spray 9x13-inch baking pan with non-stick cooking spray. Set aside.
In a large bowl, mix together the cake mix, water, vegetable oil, eggs, and vanilla. Beat on medium speed for 2 minutes. Pour cake batter into greased 9x13-inch baking dish.
Bake 28-32 minutes, until a toothpick inserted into the center comes out clean.
Remove from oven and immediately poke holes in the cake using the end of a wooden spoon.
In a medium bowl, stir together the caramel sauce and the sweetened condensed milk. Pour the mixture over the cake, making sure that it gets into all the holes. Use a spoon or spatula to spread the caramel over the cake if needed.
Crush up Butterfinger candy bars then sprinkle over the top of the cake.
Allow the cake to cool then refrigerate for at least two hours.
Spread Cool Whip over the cold cake, making sure to spread it to the edges
(Optional) Use additional crushed Butterfinger candy bars to decorate the top of the cake.