Place cake mix,butter milk eggs and vanilla in mixing bowl,blend with electric mixer for 1 min. on low. Increase speed to med. and beat 2 min.batter should be thick and well blended, pour into greased 13 by 9-inch pan, place in oven, bake for 35 min.cake should be GOLDEN brown.
while cake is baking; place pudding mixand milk in saucepan, fold in butter and vanilla, cook over low heat til butter melts, remove from heat,let cool, fold in banana slices.
Spoon pudding&banana mixture on top of cooled cake, cover the pudding with the whipped topping, sprinkle the vanilla wafers over the top.