Preheat oven to 375 F. Lightly grease 12 muffin cups or line with muffin papers.
In a large bowl, mix together 1 1/2 cups flour, baking soda, baking powder, and salt.
In another bowl, beat together bananas, sugar, egg, and melted butter.
Stir the banana mixture and optional walnuts into the flour mixture just until all are moistened.
Spoon batter into prepared muffin cups. Sprinkle crumb topping over the tops
Bake in preheated oven for 18 to 20 minutes, until a toothpick inserted into the center of a muffin, comes out clean. Err on the side of underbaking, even if the centers appear to be a little sunken.
To make crumb topping, in a small bowl, mix brown sugar, flour, and cinnamon.
Cut in butter until mixture resembles coarse cornmeal.