Preheat your oven to 180 C / Gas 4 and line a 12 hole cupcake tin with paper cases.
In a bowl, cream the butter and sugar together until light and fluffy. Add one egg at a time and beat the mixture together until smooth. Stir in the mashed bananas. Sift in the flour and baking powder and fold them into the mixture. Spoon the mixture into 12 cake cases so that each case is two-thirds full.
Bake for 20 to 25 minutes in the preheated oven until risen and golden brown and a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool before serving.