Bake cake in a 9x13 pan. While cake is still hot, pour butterscotch topping over cake, poking small holes in cake so that topping can soak in. Cool completely. Crush Butterfinger bars. Reserve half of one bar and mix the rest into a large carton of Cool Whip along with the pecans. Spread over cooled cake and top with reserved Butterfinger. Refrigerate until ready to serve.