Preheat your oven to 350°F and grease a 9x5-inch loaf pan or line it with parchment paper for easy removal.
In a large bowl, mash the bananas until mostly smooth. Whisk in the melted butter, brown sugar, granulated sugar, eggs, and vanilla until well combined.
Add the sour cream or yogurt and stir until the mixture looks creamy.
In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon. Fold the dry ingredients into the banana mixture just until no flour streaks remain.
Gently stir in the walnuts or pecans if you’re using them.
Pour the batter into the prepared loaf pan and smooth the top. Sprinkle with the optional cinnamon sugar topping.
Bake for 55 to 65 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Cool the loaf in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.