Preheat oven to 375 degrees F (190 degrees C). 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, vanilla & melted butter. Stir the banana mixture into the flour mixture just until moistened. Spoon batter into prepared muffin cups.
In a small bowl, mix together brown sugar, 2 tablespoons flour, cinnamon & nutmeg. 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.