Preheat oven to 350°F. Lightly grease and flour a 9×5-inch loaf pan and set aside.
In a large bowl, melt the butter and let cool slightly. Whisk in the sugar then the eggs and vanilla. Stir in the flour and baking soda. Fold in the mashed bananas until just combined.
In a small bowl, use a fork to stir together the cinnamon sugar swirl ingredients until thick and well combined. If your swirl is too thick to swirl, add an additional tablespoon of melted butter until it is thin enough to swirl. Note: if it’s too thin, it will just spread and not hold its swirl as the loaf bakes.
Spread 1/4 of the batter (about 1 cup) evenly into the bottom of the prepared pan. Dollop 1/4 of the cinnamon sugar mixture (about 1 tablespoon) over the batter in the pan and then make a nice thick swirl through the batter with a table knife or bottom of a wooden spoon.
Place 1/3 of the remaining batter (about 1 cup) evenly over the first layer and swirl with 1/3 of the remaining cinnamon sugar mixture (about 1 tablespoon).
Place 1/2 of the remaining batter (about 1 cup) evenly over the second layer and swirl with 1/2 of the remaining cinnamon sugar mixture (about 1 tablespoon).
Spread the remaining batter evenly over the top and dollop with the remaining cinnamon sugar mixture. Make large swirls throughout the entire loaf. Place six thin banana slices down the middle of the batter.
Place loaf pan on a baking sheet in the middle rack of the oven and bake for 60-70 minutes or until golden brown and a toothpick inserted in the center comes out clean. If the top of the bread is browned before the center is baked all the way through, tint the loaf with foil and let it continue to bake until the center is baked all the way through.
Let the bread cool in the pan for at least 30 minutes before removing to a wire rack to cool completely.
