1. Preheat the oven to 180°C (160°C fan) mark 4. Grease and line 2 20.5cm (8in) sandwich cake tins with parchment paper. In a large bowl, beat together the butter and sugar with an electric hand whisk until pale and fluffy, about 5min. Gradually beat in the eggs, then whisk in the bananas until smooth (if you don’t have electric beaters, mash the bananas with a fork and fold into the mixture). The mixture may look a little curdled, but don’t worry.