Butter a 900g loaf tin, then line with a wide strip of baking paper.
Heat the oven to 180C/fan 160C/gas 4. Beat the butter and sugar in a bowl with electric beaters for a 2-3 minutes or until fluffy. Tip in the baking powder and half the flour, half the eggs and half the bananas, and mix well (this will help prevent the mixture splitting). Add the rest of the flour, eggs and bananas, and mix to a batter.
Spoon into the loaf tin and level out. Bake for 40-45 minutes or until a skewer inserted into the middle comes out clean. Cool for 10 minutes then turn out of the tin and cool completely on a wire rack before serving.