Preheat oven to 350 degrees Fahrenheit and line a 13 x 9 x 2-inch baking dish with aluminum foil.
In a large mixing bowl, whisk together the cake mix, flour, pudding mix, and cocoa.
In another bowl whisk together the milk, oil, butter, and eggs. Add the cake mixture and stir until moistened.
Press half of the dough mixture into the bottom of the prepared pan. Bake for 10 minutes and remove from the oven.
Chop 2 Butterfinger candy bars. Stir peanut butter, and vanilla into the remaining cake mixture. Fold in the candy bar pieces along with 1 cup of chocolate chips.
Chop 3 additional candy bars and sprinkle over the top of the warm crust then press down gently. Spoon the other half of the mixed cake mixture evenly on top. Press down with a metal spatula.
Crush the last remaining candy bar on top and sprinkle with remaining chips all over the top.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool completely and cut into bars.