Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.

In a large mixing bowl, combine the mashed bananas and melted butter. Stir in the granulated sugar, brown sugar, beaten egg, and vanilla extract until well mixed.

Add the baking soda and salt, then mix in the flour until just combined. Be careful not to overmix.

In a small bowl, combine the cinnamon and sugar for the filling.

Pour half of the banana bread batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture over the batter. If using, sprinkle half of the chopped nuts on top.

Pour the remaining batter over the cinnamon layer and top with the remaining cinnamon-sugar mixture and nuts. Use a knife to gently swirl the cinnamon mixture into the batter.

Bake for 60-65 minutes, or until a toothpick inserted into the center comes out clean. Let the bread cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.