Lightly grease two baking trays.
Cream the butter or margarine with the sugar until very soft, light and fluffy.
Gradually add the egg, beating continuously, then beat in the vanilla essence.
Spread the banana chips between two pieces of greaseproof paper and crush them lightly with a rolling pin.
Stir the banana chips into the creamed mixture, together with the flour and chocolate chips.
Drop heaped teaspoonfuls of the mixture, sufficiently apart to allow room for spreading, on to the prepared baking trays and flatten slightly with the back of a fork.
Bake in a moderate oven (170C, 325F, gas 3) for 15-20 minutes, or until golden.
Leave the cookies to cool on the trays for a few minutes before transferring them to a wire rack to cool completely.
Variation Chocolate and nut chip cookies: follow the method for banana and chocolate chip cookies, but replace the banana chips with 50 g/2 oz roughly chopped walnuts.