Preheat your oven to 350°F and get your loaf pan ready with parchment paper and a bit of butter.
Melt the butter in a nonstick saucepan over medium-low heat. Once melted, turn the heat to the lowest flame, add the granulated and brown sugars, and stir for 1-2 minutes.
Take it off the heat and wait around 5 minutes. Then, add the eggs and vanilla, whisking for 1-2 minutes until you get a smooth, silky mixture.
Mix in those mashed bananas until everything's well combined.
Sift in the flour, baking soda, and salt, and gently fold until it's all mixed in.
Finally, fold in the sour cream.
Pour the batter into your prepared loaf pan and sprinkle that cinnamon sugar mixture on top.
Bake for about 50 minutes or until your banana bread is cooked through.
Let it rest for 20 minutes in the loaf pan before turning it out. Now, it's time to enjoy!