Mash the banana in a bowl. Some lumps are ok. Add the egg, sugar and melted butter. Stir to combine.
Add cinnamon. Sprinkle baking powder & soda over wet ingredients to avoid clumps, or mix with flour. Add flour and mix just until combined. Don’t overmix.
Spoon into 4 parchment-lined muffin tins, and sprinkle turbinado or cinnamon sugar generously on top.
Bake at 220° C for 5 minutes. Do not open the oven door. Lower the heat to 180° C / 350° F and bake another 12-15 minutes.