Step 1: Preheat your oven to 350°F (175°C) and grease and flour a 9×5-inch loaf pan, or line with parchment paper.
Step 2: In a large bowl, mash the very ripe bananas with a fork or potato masher until mostly smooth with a few small lumps remaining.
Step 3: Add the granulated sugar, brown sugar, melted unsalted butter, large eggs, and vanilla extract to the mashed bananas. Whisk until well combined.
Step 4: Stir in the buttermilk or sour cream.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix; a few streaks of flour are acceptable.
Step 7: If using optional mix-ins like chocolate chips or nuts, gently fold them into the batter.
Step 8: Pour the batter into your prepared loaf pan. You can sprinkle extra brown sugar or sliced banana on top for a nice crust.
Step 9: Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. If the top browns too quickly, loosely tent with aluminum foil.
Step 10: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before inverting it onto a wire rack to cool completely. Slice and serve once cooled.