Preheat the oven to 350°F. Grease a 9 by 5 inch loaf pan and line it with parchment paper.
Whisk the mashed bananas, melted butter, brown sugar, eggs, and vanilla until mostly smooth.
Stir together the flour, baking soda, baking powder, and salt. Fold the dry ingredients into the banana mixture just until combined.
Mix the cinnamon, granulated sugar, and flour for the filling.
Add half the batter to the pan, sprinkle with the cinnamon mixture, then cover with the remaining batter. Swirl gently with a knife.
Bake for 50 to 60 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
Cool for 15 minutes in the pan, transfer to a rack, and drizzle with the powdered sugar glaze once mostly cool.