Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper for easy removal.

In a large mixing bowl, combine the mashed bananas, granulated sugar, brown sugar, and melted butter. Mix well until smooth.

Add the egg and vanilla extract to the banana mixture, stirring until fully incorporated.

In a separate bowl, whisk together the baking soda, salt, and flour. Gradually add the dry ingredients to the banana mixture, stirring until just combined. Be careful not to overmix.

In a small bowl, mix the ground cinnamon with the chopped nuts (if using).

Pour half of the banana bread batter into the prepared loaf pan. Sprinkle half of the cinnamon-nut mixture over the batter. Pour the remaining batter on top and finish with the rest of the cinnamon-nut mixture. Use a knife to gently swirl the batter for a marbled effect.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.