Preheat oven to 375. Line a muffin pan with liners or spray with cooking spray.
In a large bowl, combine flour, sugar, baking powder, salt, baking soda, and cinnamon. In a small bowl beat together eggs, sour cream, butter and bananas. Stir into the dry ingredients until moistened. Add in chopped pecans or walnuts. Fill the muffin cups 2/3 of the way full.
To make the cinnamon streusel: In a small bowl combine brown sugar, flour, cinnamon and cut in butter until crumbly. Sprinkle on top of the muffins and bake for 20-25 minutes or until toothpick comes out clean and they are slightly golden. Makes about 12 muffins.