Preheat your oven to 350°F (175°C) and grease a 9x5-inch loaf pan with butter or non-stick spray.

In a large mixing bowl, combine mashed ripe bananas with melted unsalted butter. Stir in granulated sugar, beaten egg, and vanilla extract.

In a separate bowl, whisk together all-purpose flour, baking soda, and a pinch of salt. Gradually add the dry mixture to the wet ingredients, mixing gently.

In a small bowl, mix brown sugar with ground cinnamon.

Pour half of the banana batter into the greased loaf pan, sprinkle half of the cinnamon-sugar mixture on top, then add the remaining banana batter, finishing with the rest of the cinnamon mixture.

Gently swirl through the batter with a butter knife or skewer for a marbled effect.

Bake in the preheated oven for 60–70 minutes, checking for doneness at the 60-minute mark.

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