Preheat oven to 350 degrees. In a large mixing bowl, combine cake mix, flour, pudding mix and cocoa. In another bowl whisk together milk, oil, butter and eggs. Add to the dry ingredients and stir until moistened.
Line a 9x13 with aluminum foil. Press half of the dough mixture into the bottom. Bake for 10 minutes.
While baking chop 2 Butterfinger candy bars. Stir peanut butter, and vanilla into the other half of the uncooked cake mixture. Fold in chopped bars and 1 cup chocolate chips.
Chop 3 additional candy bars and sprinkle over the top of the warm crust and press down gently. Drop 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 toothpick inserted in center comes out clean. Cool completely and cut into bars.