1. Preheat to 375 degrees F (190 degrees C). Lightly grease 10 muffin cups, or line with muffin papers.
2. 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 oil (vanilla and walnuts if desired). Stir banana mixture into the flour mixture just until moistened. Spoon batter into prepared muffin cups.
3. In a small bowl, mix together brown sugar, 2 tablespoons flour and cinnamon. Cut in 1 tablespoon butter until it is a streusel consistency. Sprinkle topping over muffins. 4. Bake in preheated oven for 18 to 20 minutes.