Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan with butter or non-stick spray.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, cream the softened unsalted butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas until combined.
Add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
In a small bowl, combine the brown sugar and ground cinnamon to make the cinnamon sugar swirl.
Pour half of the banana bread batter into the prepared loaf pan. Sprinkle half of the cinnamon sugar mixture evenly over the batter.
Pour the remaining batter over the cinnamon sugar layer, then sprinkle the remaining cinnamon sugar on top.
Using a butter knife or skewer, gently swirl the cinnamon sugar through the batter to create a marbled effect.
Bake in the preheated oven for 55 to 65 minutes, or until a toothpick inserted into the center comes out clean.
Remove the bread from the oven and let it cool in the pan for 10 minutes.
Transfer the bread to a wire rack and cool completely before slicing.