Preheat oven to 375°F and line a muffin tin with cupcake liners (or spray the cups generously with cooking spray). Using a stand mixer fitted with the paddle attachment (or a large mixing bowl with a hand mixer), combine the butter, sugar and egg and beat until creamy.
In a separate bowl, combine the flour, baking powder, baking soda and salt. Mix well. Gradually add to the butter mixture and beat on medium-low speed until just combined. Add the mashed bananas and mix until just combined.
Prepare the crumb topping: In a small bowl, combine brown sugar, flour, cinnamon and melted butter. Using fork tines, mix until crumbly.
Divide the batter between the prepared muffins cups. The batter will be thick and the cups should be 3/4 of the way full. Cover with the crumb topping. Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the centers comes out clean. Serve warm!