In a saucepan, combine flour, egg yolks, sugar, and milk. Mix until smooth. Bring slowly to a boil and cook until thick and the consistency of pudding. Stir in vanilla. In a 2-quart casserole dish, place a layer of vanilla wafers, a layer of thinly sliced bananas, and about half of the pudding. Repeat, working quickly because the heat of the pudding turns this into a cakelike mixture.