Preheat your oven to 350°F (175°C). Grease and flour a 9×5 inch loaf pan.
In a medium bowl, whisk together the flour, baking soda, and salt.
In a separate large bowl, cream together the softened butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk (or sour cream), beginning and ending with the dry ingredients. Mix until just combined.
In a small bowl, combine the sugar, cinnamon, and melted butter. Mix until it forms a thick paste.
Pour half of the banana bread batter into the prepared loaf pan. Sprinkle the cinnamon swirl mixture evenly over the batter. Pour the remaining batter over the cinnamon swirl.
Use a knife or skewer to gently swirl the cinnamon mixture into the batter. Be careful not to overmix.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.