1. Pre-heat the oven to 160ºC and line a loaf tin with baking paper. Cream the butter with the sugar in a bowl or in a food processor.
2. Add the eggs one at a time and then the vanilla. If it curdles, add a handful of the flour.
3. Fold in the flour and mashed banana (this can all be done in a food processor until smooth). Pour the mixture into the loaf tin.
4. Bake for 1 hour and check that it's cooked through by inserting a skewer into the centre to see if it comes out clean. This cake will keep for a few days in an airtight container.
Tip: You can add chopped walnuts or chocolate drops before baking for variation.
Recipe courtesy of www.yakult.co.uk