The bananas need to be super ripe for this cake recipe. If your bananas are under-ripe, here is a neat trick to help. Simply heat your oven to 200°C (fan 180°C/400°F/gas 6) and place the bananas (in their skins) on a baking sheet. Bake them for about 15–20 minutes, or until the skins have turned black. Allow to cool a little, then peel back the banana skin to reveal soft, squishy and slightly sweet bananas. Now, will it be one slice or two?