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, salt, and 1 teaspoon ground cinnamon. Set aside.
In a large bowl, cream the softened butter and 1 cup granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed bananas and sour cream until just combined.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.
In a small bowl, combine 2 teaspoons ground cinnamon and 2 tablespoons granulated sugar to create the cinnamon-sugar topping.
Pour half of the 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 mixture on top.
Using a knife or skewer, gently swirl the cinnamon-sugar topping into the batter to create a marbled effect.
Bake for 55-60 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 let it cool completely before slicing.