Mash the bananas, this is easiest when the bananas are overripe, grind the oatmeal in a blender and let the butter melt in the microwave at 600 watts for 1 minute.
Mix all ingredients in a large bowl. Grease a cake tin and spread the batter in it. Bake at 180 degrees for about 50 minutes in a preheated oven with convection.