Preheat the oven to 350°F. Lightly spray a 9x13-inch glass baking dish with baker's spray (like Baker’s Joy brand). Set aside.
In a medium-sized bowl with a pour spout (a large glass measuring cup works great), stir together the sweetened condensed milk and jarred caramel sauce until completely combined. Set the caramel mixture aside.
In a large mixing bowl, whisk together the ingredients for the box cake mix for 1 to 2 minutes or just until completely combined and no lumps remain.
Bake the cake for 28 to 30 minutes or until lightly golden and a toothpick, inserted into the center of the cake, comes out clean.
When the cake comes out of the oven, using a fork or wooden skewer, poke lots of holes all over the entire cake. Slowly pour the caramel filling mixture evenly over the top of the warm cake. Make sure to do this slowly so that your sauce gets evenly distributed into the warm cake and does not pool up in one area of your cake.
Sprinkle ½ cup of the crushed Butterfinger candies evenly over the top of the cake. Allow your cake to cool on the counter for 1 hour.
Evenly spread the Cool Whip onto the cooled cake and top with the additional 1 cup of crushed Butterfinger candy pieces.
Refrigerate the Butterfinger cake for 1 to 2 hours, up to overnight, before slicing and serving.