Preheat your oven to 350°F (175°C). Grease a 9×5 inch loaf pan or line it with parchment paper.
In a large mixing bowl, combine the mashed bananas and melted butter. Stir until well blended.
Add the granulated sugar, eggs, and vanilla extract to the banana mixture. Mix until smooth and fully incorporated.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon.
Gradually add the dry ingredients to the wet ingredients, stirring gently just until combined. Do not overmix.
Fold in the sour cream to the batter until evenly distributed.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake in the preheated oven for 55 to 60 minutes, or until a toothpick inserted into the center comes out clean.
Remove the banana bread from the oven and let it cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing and serving.