Preheat oven to 350°. Spray a 13x9-inch baking pan with nonstick baking spray with flour.
Prepare cake mix according to package directions, stirring in 1 cup chopped Butterfinger to batter. Pour into prepared pan, and bake according to package directions.
Let cake cool for 5 minutes. Using the handle of a wooden spoon, poke holes in cake in 1-inch intervals. Pour condensed milk and caramel topping evenly onto cake. Sprinkle with remaining 1 chopped Butterfinger. Cover and refrigerate overnight.
Just before serving, spread whipped topping onto cake. Garnish with chopped Butterfinger, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/butterfinger-cake-recipe/