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

Mash 3 ripe bananas in a bowl and mix in ⅓ cup melted butter, ¾ cup sugar, 1 beaten egg, and 1 teaspoon vanilla.

In another bowl, whisk together 1½ cups flour, 1 teaspoon baking soda, and ¼ teaspoon salt. Fold into the wet mix until just combined.

Combine 1 tablespoon ground cinnamon with ¼ cup brown sugar in a small bowl.

Layer half of the batter in the loaf pan, sprinkle half of the cinnamon-sugar mix, then add the remaining batter and top with the rest of the cinnamon mixture.

Use a knife to gently swirl through the batter to create marbling.

Bake in the preheated oven for 60-70 minutes, or until a toothpick comes out clean.

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