Preheat the oven to 180°C (160°C fan, Gas 4). Grease a 900g (2lb) loaf tin and line with baking parchment.
Mash the bananas in a separate bowl using a fork.
Put the butter, sugar, eggs, mashed bananas, flour, baking powder and milk into a large mixing bowl. Beat together for about 2 minutes until smooth and well combined.
Spoon the mixture into the prepared tin and level the top. Sprinkle over the Demerara sugar if using.
Bake for 55-65 minutes, or until well risen, golden, and a skewer inserted into the centre comes out clean.
Cool in the tin for 10 minutes, then turn out onto a wire rack to cool completely.