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 sift in the flour and baking powder, mixing until all combined.
Pour into your pre-lined loaf tin and smooth into the corners.
Bake in your pre-heated oven for 40 minutes until golden on top. Leave to cool in the loaf tin before serving in slices.