Melt butter, sugar and vanilla extract in a saucepan over a medium heat. Remove from heat and add the mashed bananas, mix well.
Add the egg, mix well and stir in the flour and milk and beat with a hand mixer until you have a smooth batter.
Pour the mixture into the loaf tin, if desired, sprinkle with a tablespoon of demerara sugar to give a crunchy topping.
Select the cake option and leave until the programme has ended. To check it is cooked, place a clean skewer in the centre of the cake, if there is residue on the skewer then pop the cake onto a baking tray and pop into the oven for 5-10 minutes.
Leave to cool and enjoy!