1. Preheat the oven to 375°F (190°C). Line a muffin tin with paper liners.
2. For the muffins:
In a bowl, mix 1 1/2 cups of flour, baking soda, baking powder, and salt.
In another bowl, beat mashed bananas, sugar, lightly beaten egg, and melted butter. Incorporate this mixture into the flour. Divide the batter into the muffin cups.
3. For the topping:
In a bowl, mix brown sugar, 2 tablespoons of flour, and cinnamon. Crumble 1 tablespoon of butter into this mixture to make a crumbly paste. Sprinkle this paste over the muffins.
4. Bake for 18 to 20 minutes. A toothpick inserted into the center should come out clean. Allow to cool before serving.