Pre-heat the oven to 160C(320F) fan-forced. Grease a loaf pan and line it, extending the paper over the two long sides.
Combine butter, banana, sugar, eggs and maple syrup in a large bowl.
Stir in the dry ingredients and the walnuts then transfer the mixture to the pan.
Bake for about 1 hour or until a tester comes out clean.
Cool on a wire rack.