Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a mixing bowl, mash the bananas until smooth and stir in the melted butter.
Add the baking soda, salt, and sugar to the banana mixture, then mix in the beaten egg and vanilla extract.
Gradually add the flour to the banana mixture and fold it in gently.
In a separate bowl, combine the brown sugar, cinnamon, and flour, then mix in the softened butter until it resembles moist crumbs.
Pour half of the banana batter into the prepared loaf pan, sprinkle half of the cinnamon swirl mixture, add the remaining banana batter, and then sprinkle the rest of the cinnamon swirl mixture.
Use a knife or skewer to swirl the batter and cinnamon mixture together.
Bake for approximately 60-70 minutes, or until a toothpick comes out clean.
Remove from the oven and cool in the pan for about 10 minutes before transferring to a wire rack.