Preheat your oven to 350°F (175°C). Line a muffin pan with paper liners or grease lightly.
In a bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
In a separate bowl, mix mashed bananas, sugars, melted butter, egg, and vanilla until combined.
Add dry ingredients to wet ingredients and stir just until combined. Fold in chopped nuts.
Prepare the streusel by mixing flour, brown sugar, cinnamon, and cold butter with a fork or fingers until crumbly.
Divide the batter evenly into the muffin cups. Sprinkle streusel generously on top.
Bake for 20–22 minutes or until a toothpick inserted into the center comes out clean.
Cool for 5 minutes in the pan, then transfer to a rack.