Peel and cut the bananas into chucks about 2cm thick. Add them to a bag and freeze overnight.
When you are ready for the ice-cream, add the frozen banana chunks to a food processor. Add the vanilla extract, sugar and 75ml of the buttermilk and blitz.
After about 20 seconds and while the food processor is still blending, add the remaining buttermilk in a continuous slow pour. Continue blending until you have a smooth and creamy mixture.
Scoop out the mixture and serve immediately.