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

In a large mixing bowl, mash 3 ripe bananas until smooth, then add in 1/2 cup of melted unsalted butter, 1 cup of granulated sugar, 1 beaten egg, and 1 teaspoon of vanilla extract. Mix well.

In a separate bowl, whisk together 1 1/2 cups of all-purpose flour, 1 teaspoon of baking soda, 1/2 teaspoon of salt, and 2 teaspoons of ground cinnamon.

Gradually fold the dry ingredients into the wet mixture until just combined, being careful not to over-mix.

In a small bowl, combine 1/4 cup of brown sugar and 2 teaspoons of ground cinnamon for the swirl mixture.

Pour half of the banana bread batter into the greased pan, sprinkle half of the cinnamon sugar mixture, then spoon the remaining batter, followed by the rest of the cinnamon mixture on top.

Using a knife, gently swirl the cinnamon mixture through the layers of batter.

Bake for 60-70 minutes, checking with a toothpick for doneness.

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