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 1/2 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.