Preheat your oven to 350°F (175°C).
Lightly grease your muffin tin or line it with paper liners.
In a large mixing bowl, whisk together the dry ingredients: flour, sugar, baking powder, salt, baking soda, and cinnamon.
In a separate bowl, mash the ripe bananas, then add eggs, sour cream, and melted butter. Whisk until combined.
Pour the wet mixture into the dry ingredients and gently fold together until just combined.
If using, fold in chopped pecans or walnuts.
In a small bowl, mix together brown sugar, flour, cinnamon, and softened butter to create the streusel topping.
Scoop the batter into the muffin tin, filling each cup about two-thirds full.
Sprinkle the streusel topping over each muffin.
Bake for 18-20 minutes, or until a toothpick comes out clean.
Let cool in the tin for 5 minutes before transferring to a wire rack.
