Preheat your oven to 350°F (175°C). Grease a 9x5 inch loaf pan.
In a mixing bowl, whisk together the flour, baking soda, and salt.
In another bowl, mix the melted butter and brown sugar until well combined. Add the beaten eggs, vanilla extract, and sour cream. Mix well.
Stir in the mashed bananas.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
In a small bowl, mix the granulated sugar and ground cinnamon for the swirl.
Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture over the batter. Pour the remaining batter on top and sprinkle with the remaining cinnamon-sugar mixture. Use a knife to swirl the cinnamon-sugar through the batter.
Bake for 60 minutes, or until a toothpick inserted into the center comes out clean. Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.