Preheat the oven according to the temperature listed on the chocolate cake mix package.
Lightly grease a 9x13-inch baking dish with nonstick cooking spray.
Place the chocolate cake mix in a large mixing bowl.
Add the whole milk, canola oil, and eggs.
Mix until the batter is smooth and fully combined.
Pour the batter into the prepared baking dish.
Spread the batter evenly in the dish.
Bake according to the instructions on the cake mix package, or until a toothpick inserted into the center comes out clean.
While the cake is baking, combine the sweetened condensed milk and caramel dessert topping in a medium mixing bowl.
Stir until the mixture is smooth and evenly combined.
Remove the baked cake from the oven.
While the cake is still hot, use the handle of a wooden spoon to poke holes across the surface of the cake, spacing them about 1 inch apart and making them approximately 1 inch deep.
Pour the sweetened condensed milk and caramel mixture evenly over the hot cake.
Spread the mixture gently over the surface, allowing it to fill as many holes as possible.
Allow the cake to cool completely.
Once the cake has cooled, spread the thawed whipped topping evenly over the entire surface.
Sprinkle the crushed Butterfinger candy bars evenly over the whipped topping.
Drizzle with additional caramel sauce, if desired.
Refrigerate the cake until ready to serve.
Slice and serve chilled.