Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan.

In a mixing bowl, combine 3 mashed ripe bananas, 1/2 cup melted unsalted butter, 1 cup granulated sugar, 1 beaten egg, and 1 teaspoon vanilla extract. Stir until blended.

In a separate bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking soda, and 1/2 teaspoon salt. Gradually mix into the wet ingredients until just combined.

In a small bowl, mix 1/4 cup brown sugar and 2 teaspoons ground cinnamon to create the swirl mixture.

Pour half of the banana batter into the greased loaf pan, sprinkle half of the cinnamon-sugar mixture, followed by the remaining batter and the rest of the cinnamon mixture.

Gently swirl the cinnamon mixture into the top layer of batter using a knife or skewer.

Bake for 60-70 minutes, checking doneness with a toothpick. If needed, cover with foil to prevent over-browning.

Let the bread cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.