Preheat your oven to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper.
In a small bowl, mix together 1/4 cup sugar and 1 tablespoon cinnamon for the cinnamon-sugar swirl. Set aside.
In a medium bowl, whisk together flour, baking soda, baking powder, salt, and 1 teaspoon cinnamon.
In a large bowl, beat the softened butter and 3/4 cup sugar together until light and fluffy.
Add eggs one at a time, mixing well after each addition.
Stir in vanilla extract, mashed bananas, and sour cream until combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined to avoid overmixing.
Pour half the batter into the prepared loaf pan. Sprinkle half the cinnamon-sugar swirl over the batter.
Add the remaining batter, then sprinkle the rest of the cinnamon-sugar swirl on top. Use a knife to gently swirl the mixture into the batter.
In a small bowl, mix 2 tablespoons sugar and 1 teaspoon cinnamon for the snickerdoodle topping. Sprinkle evenly over the top of the batter.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.