Preheat your oven to 350°F (175°C) and prepare a loaf pan by greasing it or lining with parchment paper.

In a large mixing bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth.

Incorporate sour cream, then add eggs one at a time, blending well after each.

Stir in vanilla extract and mashed bananas until fully combined.

In a separate bowl, mix all-purpose flour, baking soda, baking powder, salt, and ground cinnamon until no lumps remain.

Sprinkle the dry mixture over the wet banana batter and fold gently until just combined.

Pour the batter into the prepared loaf pan, smoothing the top, and add chocolate chips or nuts if desired.

Bake in the preheated oven for 45-55 minutes, checking for doneness with a toothpick.

Cool in the pan for 10-15 minutes before transferring to a wire rack.