PREPARE cake according to package directions using a 13 x 9-inch baking pan. Cool completely in pan on wire rack.
POKE holes in cake about 1-inch apart and 1-inch deep using handle of wooden spoon. Pour sweetened condensed milk over cake. Sprinkle with half of chopped Butterfinger. Pour caramel topping over Butterfinger.
SPREAD whipped cream over cake; sprinkle with remaining chopped Butterfinger. Store in refrigerator until ready to serve. Refrigerate any uneaten cake.