Mix the mashed bananas and melted butter in a large bowl. Stir in the granulated sugar, egg, and vanilla until smooth.
Fold the flour, baking soda, and salt into the wet mixture gently just until the dry streaks disappear.
Mash the brown sugar, cinnamon, and softened butter together in a small bowl until it resembles wet sand to create the swirl filling.
Pour half of the batter into a greased loaf pan. Sprinkle the cinnamon sugar mixture evenly over it, then cover with the remaining batter.
Bake at 350 degrees Fahrenheit for 50 to 60 minutes until a skewer comes out clean. Cool completely before whisking the powdered sugar and milk together to drizzle over the top.
