Preheat the oven to 350°F (175°C). Grease a 9×5 inch loaf pan and set aside.
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 using an electric mixer.
Add the eggs one at a time to the butter and sugar mixture, beating well after each addition.
Mix in the vanilla extract.
Stir in the mashed ripe bananas, sour cream, and milk until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
In a small bowl, combine the 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 banana bread batter over the cinnamon sugar layer and spread evenly.
Sprinkle the remaining cinnamon sugar mixture evenly on top.
Using a knife or skewer, gently swirl the cinnamon sugar into 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 allow it to cool in the pan for 10 minutes.
Transfer the bread to a wire rack and let it cool completely before slicing.