Preheat the oven to 180 C. Put 12 paper baking cases in a muffin tin.
Sift and stir together the flour, baking powder, bicarbonate of soda and salt then leave it, we'll come back to it later. Stir together the mashed bananas, sugar, egg and butter in a big bowl. Stir the mixture we already made (with the flour in it) and mix it in with the other mixture (the banana one) until it is smooth.
Spoon evenly into the baking cases in the muffin tin.
Bake in the preheated oven for 25 to 30 minutes or until muffins are golden. WEAR OVEN GLOVES! Get and adult's help.