Cream margarine and sugar until light and pale in colour.
Add egg and flour a little at a time. Try not to add egg too quickly as it will curdle. Slowly add banana. Spoon mixture into muffin cases.
Place in preheated 190 C / Gas 5 oven and bake for about 20 minutes, or until cakes are golden and spring back when pressed on top. (Should not leave a finger print.)