Preheat oven to 375 degrees F (190 degrees C). Lightly grease 10 muffin cups, or line with muffin papers.
In a large bowl, mix together the melted butter, mashed bananas, sugar and egg. Add to the mix: 1 1/2 cups flour, baking soda, baking powder and salt. Stir until just moistened. Spoon batter into prepared muffin cups.
In a small bowl, mix together brown sugar, 2 tablespoons flour and cinnamon. Cut in 1 tablespoon butter until mixture resembles coarse cornmeal. Sprinkle topping over muffins.
Bake in preheated oven for 18 to 20 minutes, until a toothpick inserted into center of a muffin comes out clean.