Preheat your oven to 350 degrees Fahrenheit and grease a 9x5 inch loaf pan.
Mix the mashed bananas, melted butter, granulated sugar, egg, and vanilla extract in a large bowl.
Fold in the 1.5 cups of flour, baking soda, and salt gently until just combined.
Combine the brown sugar, cinnamon, and 1 tablespoon of flour in a small dish for the swirl filling.
Pour half the batter into the pan, top evenly with the swirl mixture, then spoon the remaining batter over the top. Marble with a butter knife.
Bake for 50 to 60 minutes. Let cool, then whisk the powdered sugar and milk together and drizzle over the top.