1. Preheat your oven to 350°F and line a 9×5-inch loaf pan with parchment paper, leaving overhang on the long sides. Grease the parchment lightly with butter or cooking spray.

2. In your large mixing bowl, mash the very ripe bananas with a fork until mostly smooth. Add the ⅓ cup melted butter and stir together, then add the granulated sugar, beaten egg, and 1 teaspoon vanilla extract, mixing until well combined and creamy.

3. Sprinkle the baking soda and salt over the wet mixture and stir well to distribute evenly. Add the flour all at once and gently fold it in, mixing just until no dry flour streaks remain.

4. In a small bowl, combine the brown sugar, cinnamon, and 2 tablespoons melted butter, stirring with a fork until it forms a thick, paste-like mixture.

5. Pour half of the banana bread batter into your prepared loaf pan and spread evenly. Dollop half of the cinnamon mixture over the batter, then use a butter knife to swirl it through with gentle figure-eight motions. Pour the remaining batter on top and spread gently, then dollop the remaining cinnamon mixture on top and swirl again.

6. Bake for 60-70 minutes, until the top is golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. Check in a couple of spots since the cinnamon swirl can make doneness tricky.

7. Let the bread cool in the pan for 15 minutes, then lift onto a wire rack. While cooling, whisk together the powdered sugar, milk, and ½ teaspoon vanilla extract until smooth. Drizzle the glaze over the warm bread and let it set for 10 minutes before slicing.