Preheat oven to 375 degrees Fahrenheit. Line a muffin pan with cupcake liners or spray with non-stick cooking spray.
Combine 2 cups all-purpose flour, 1 cup granulated sugar, 1 teaspoon baking powder, ½ teaspoon salt, ½ teaspoon baking soda, and ¼ teaspoon ground cinnamon in a large bowl. Set aside.
In a medium bowl add 2 eggs, 1 cup sour cream, ¼ cup melted butter, and 2 ripe mashed bananas, Beat together using a hand mixer until smmoth.
Stir in the flour mixture until moistened. Add in ½ cup chopped pecans or walnuts, if using. Fill the muffin cups ⅔ of the way full.
In a small bowl, combine ½ cup brown sugar, 3 tablespoons all-purpose flour and ¼ teaspoon ground cinnamon. Using the back of a fork, cut in 4 tablespoons softened butter until crumbly.
Sprinkle on top of the muffins and bake for 20-25 minutes or until a toothpick comes out clean and they are slightly golden.