PREHEAT oven to 350° F. Grease and flour 13 x 9-inch baking pan.
PREPARE cake mix batter according to package directions; stir in banana and 3/4 cup chopped Butterfinger. Pour into prepared pan.
BAKE for 40 to 50 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack.
FROST cake; sprinkle with remaining chopped Butterfinger.