Prepare the pan and oven: Preheat the oven to 350°F (175°C). Grease a 9-by-5-inch loaf pan and line the bottom with parchment paper, leaving some overhang for easy removal.

Mash the bananas: Place the peeled bananas in a large mixing bowl and mash with a fork until mostly smooth. A few small pieces are fine, but avoid leaving large chunks.

Mix the wet ingredients: Add the melted, slightly cooled butter, granulated sugar, eggs, vanilla extract, and sour cream to the mashed bananas. Whisk until smooth and evenly combined.

Combine the dry ingredients: In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and cinnamon.

Make the batter: Add the dry ingredients to the banana mixture. Fold with a rubber spatula just until no obvious dry flour remains. Do not overmix.

Fill the loaf pan: Transfer the batter to the prepared loaf pan and spread it evenly. Smooth the top with the spatula.

Bake: Bake for 50–60 minutes, rotating the pan once if your oven browns unevenly. The top should be deep golden brown and the center should feel set.

Cool before slicing: Let the loaf cool in the pan for 10 minutes, then lift it out using the parchment and transfer it to a wire rack. Cool for at least 15 more minutes before slicing.