Pre-heat your oven to 190°c (170°c for a fan-assisted oven or Gas Mark 5) and grease & line a 2lb loaf tin with grease-proof paper. In a large bowl, mash the bananas with a fork. Add the sugar, butter/margarine, egg & vanilla extract and mix until combined.
Gradually add the flour and baking powder, mixing until all combined.
Pour half the batter into your pre-lined loaf tin and smooth into the corners.
Warm your Nutella in the microwave for about 10-20 second to soften it slightly. Drizzle a couple of spoonfuls over the batter in the loaf tin before adding the remaining batter on top.
Drizzle the remaining Nutella to the top of your loaf before stirring the batter with a skewer to create the marbled affect.
Bake in your pre-heated oven for 40 minutes until golden on top. Leave to cool in the loaf tin and serve in slices.