Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper.
Step 2: In a large bowl, mash the very ripe bananas until smooth with a few small lumps remaining.
Step 3: Add the melted butter, granulated sugar, brown sugar, beaten eggs, vanilla extract, and sour cream to the mashed bananas. Whisk until just combined.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon (if using).
Step 5: Add the dry ingredients to the wet ingredients. Mix with a rubber spatula or wooden spoon until *just combined*. Be careful not to overmix; a few streaks of flour are acceptable.
Step 6: Pour the batter into the prepared loaf pan, spreading evenly.
Step 7: Bake for 55-65 minutes, or until a wooden skewer inserted into the center comes out clean or with moist crumbs attached. If the top browns too quickly, loosely tent with foil.
Step 8: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes. Then, carefully remove it from the pan and transfer to a wire rack to cool completely before slicing and serving.