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

Mash the ripe bananas in a mixing bowl until mostly smooth, leaving some small lumps.

Stir in the melted butter until fully incorporated, then mix in baking soda and salt.

Add the granulated sugar, beaten egg, and vanilla extract, mixing until smooth.

Gradually fold in the all-purpose flour until just combined, being careful not to overmix.

In a separate bowl, whisk together the ground cinnamon and brown sugar.

Pour half of the banana batter into the loaf pan, followed by half of the cinnamon-sugar mixture.

Pour in the remaining banana batter, smoothing it out, then top with remaining cinnamon-sugar mixture.

Gently swirl the batters together in a figure-eight motion to create a marbled effect.

Bake in the preheated oven for 60-65 minutes, checking for doneness with a toothpick.

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