Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.

In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and mashed bananas until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix. Fold in the sour cream and nuts if using.

Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.

In a small bowl, mix the brown sugar and ground cinnamon for the topping. Sprinkle the mixture evenly over the muffin batter.

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

Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.