Preheat the oven to 350°F (175°C). Grease a 9×5 inch loaf pan or line it with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, combine the melted butter and granulated sugar. Stir until smooth.
Add the eggs one at a time to the butter and sugar mixture, beating well after each addition.
Stir in the vanilla extract.
Add the mashed bananas and buttermilk to the wet mixture and mix until combined.
Gradually add the dry ingredients to the wet ingredients, stirring just until combined. Do not overmix.
In a small bowl, mix the light brown sugar and ground cinnamon to create the cinnamon sugar swirl.
Pour half of the banana bread batter into the prepared loaf pan and spread evenly.
Sprinkle half of the cinnamon sugar mixture evenly over the batter layer.
Pour the remaining batter over the cinnamon sugar layer and spread evenly.
Sprinkle the remaining cinnamon sugar mixture on top.
Using a knife or skewer, gently swirl the cinnamon sugar into the batter to create a marbled effect.
Bake 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 to cool completely before slicing.