Preheat your oven to 350°F (175°C).

Grease your muffin tin or line it with paper liners.

In a large mixing bowl, mash the bananas until smooth. Add the granulated sugar, brown sugar, and melted butter. Mix until combined. Stir in the egg and vanilla extract until well blended.

In a separate bowl, whisk together the baking soda, baking powder, salt, flour, rolled oats, and cinnamon.

Gradually add the dry ingredients to the banana mixture, stirring gently until just combined, being careful not to overmix.

If using, fold in the chopped nuts.

Scoop the batter into the prepared muffin tin, filling each cup about 2/3 full.

In a small bowl, mix together additional flour, sugar, and butter to create a crumbly topping. Sprinkle this over the muffins.

Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool for a few minutes before transferring them to a wire rack.