Preheat the oven to 350°F (175°C).
Spray a 13 x 9-inch baking pan with nonstick baking spray that includes flour and set aside while you prepare the cake.
Combine the cake mix, flour, buttermilk, eggs and oil in a large mixing bowl. Stir just until the mix is incorporated with the wet ingredients.
Chop up one Butterfinger candy bar and mix it into the cake batter. Stir well so the pieces are evenly distributed.
Pour the prepared batter into the baking pan. Place the pan in the preheated oven and bake according to the time on the cake mix package (usually 25-30 minutes). The cake is done when a toothpick inserted into the center comes out clean.
After baking, let the cake cool in the pan for 5 minutes. Then, using the handle of a wooden spoon, poke holes all over the cake, spacing them about 1 inch apart. Make sure to poke about halfway through the cake.
Mix the sweetened condensed milk and caramel topping in a large measuring cup. Pour evenly over the cake, making sure they soak into the holes you just made. Then, sprinkle the remaining chopped Butterfinger over the top.
Cover the cake with plastic wrap or aluminum foil and place it in the refrigerator to chill for at least two hours.
Before serving, spread a layer of whipped topping evenly over the entire cake.
If you like, sprinkle some extra chopped Butterfinger pieces on top of the whipped topping for added crunch and flavor.