In a large bowl, sieve the flour, baking powder, bicarbonate of soda and salt.
In a small bowl, combine the bananas, egg, sugar and butter and mix throughly with a wooden spoon or in a stand mixer for 2/3 minutes.
Fold the banana mixture into the flour and mix till combined.
Spoon the mixture evenly between 12 cases.
Cinnamon topping
For topping combine all the ingredients for the topping.
Crumble evenly over each muffin
Put the tray into the preheated oven for 25 minutes until a toothpick inserted in the middle comes out clean.
Once baked, remove from the oven and allow to cool slightly in the muffin tin. Then, remove each muffin and allow to cool on a wire rack or in a plate.