Preheat your oven to 350°F (175°C). Grease a standard 9x5-inch loaf pan with butter or nonstick spray, then lightly dust with flour. Set the pan aside while you mix everything together.

In a large mixing bowl, mash your ripe bananas with a fork until mostly smooth. A few small lumps are fine — they add lovely texture. The riper the bananas, the sweeter and more flavorful the bread will be.

Stir the melted butter into the mashed bananas until combined. Add the brown sugar and mix well until the mixture darkens into a rich, caramel-scented batter. Add the eggs one at a time, then stir in the vanilla extract and sour cream.

Sprinkle the flour, baking soda, salt, and cinnamon directly over the wet mixture. Using a spatula or wooden spoon, fold everything together gently until just combined. Stop mixing the moment you no longer see dry streaks of flour — do not overmix.

Pour the batter into your prepared loaf pan and smooth the top. Place on the center rack of the preheated oven and bake for 55–60 minutes. After about 50 minutes, insert a toothpick into the center — it should come out clean or with just a few moist crumbs. The top should be deep golden brown and slightly domed.

Let the loaf cool in the pan for 10 minutes before turning it out onto a wire rack to cool completely.