Adjust the oven rack to the lower third position and preheat the oven to 350°F. Grease a 9 x 5 inch loaf pan.
In a large bowl, stir together the mashed bananas, butter, eggs and vanilla extract until combined. Add the flour, dark brown sugar, baking soda and salt and stir until just combined.
To make the swirl, mix together the unsalted butter, dark brown sugar and cinnamon in a small bowl until thick and combined.
Spread ⅓ of the batter evenly into the bottom of the pan. Dollop about ⅓ of the swirl mixture over the batter and use a knife to make a swirl through it. Repeat the process 2 more times. Make large swirls throughout the loaf.
Bake for 50-60 minutes, covering with aluminum foil halfway through if bread is browning too quickly. At 50 minutes, start checking to see if bread is done every 5 minutes by inserting a toothpick or cake tester in the center where it comes out clean with just a few moist crumbs.
Let the loaf cool in the pan for 30 minutes before removing and placing on wire rack.