Preheat your oven to 350°F (175°C). This ensures that the bread bakes evenly and rises properly.

Grease a 9x5 inch loaf pan with butter or non-stick cooking spray. This prevents the bread from sticking.

In a large mixing bowl, mash the ripe bananas with a fork until smooth. The riper the bananas, the sweeter and more flavorful your bread will be.

Add the melted butter to the mashed bananas and mix well. This adds moisture and richness to the bread.

Stir in the egg and vanilla extract until fully combined.

In a separate bowl, whisk together the granulated sugar, baking soda, and salt.

Gradually add the dry ingredients to the banana mixture, folding gently with a rubber spatula until just combined. Be careful not to overmix, as this can lead to a dense bread.

Finally, fold in the sifted all-purpose flour until no streaks remain. The batter should be thick and slightly lumpy.

In a small bowl, combine the brown sugar and ground cinnamon. This will create the sweet swirl that runs through the bread.

Pour half of the banana bread batter into the prepared loaf pan, spreading it evenly.

Sprinkle half of the cinnamon sugar mixture over the batter, then add the remaining banana bread batter on top.

Finally, sprinkle the remaining cinnamon sugar mixture over the top of the batter. Use a knife to gently swirl the mixture into the batter, creating a marbled effect.

Bake in the preheated oven for 55-65 minutes, or until a toothpick inserted into the center comes out clean. The top should be golden brown.

Once baked, remove from the oven and let the bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.