Prep the pan and oven: Heat oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with a parchment sling for easy removal.
Mash the bananas: In a large bowl, mash the bananas with a fork until mostly smooth with a few small lumps.
You want about 1 1/2 cups.
Whisk in the wet ingredients: Add eggs, oil, melted butter (if using), brown sugar, granulated sugar, sour cream or yogurt, and vanilla. Whisk until well combined.
Combine the dry ingredients: In a separate bowl, whisk flour, baking soda, baking powder, salt, and cinnamon. This spreads the leavening evenly.
Fold gently: Add the dry ingredients to the wet.
Stir with a spatula until just combined and no dry streaks remain. Do not overmix.
Add-ins (optional): Fold in nuts or chocolate chips. A handful goes a long way; too many can weigh down the loaf.
Fill the pan: Scrape the batter into the prepared pan and smooth the top.
For a pretty split, run a buttered knife down the center of the batter.
Bake: Bake 55–65 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. If the top browns too fast, tent loosely with foil for the last 10–15 minutes.
Cool properly: Let the loaf cool in the pan for 15 minutes, then lift it out and cool completely on a rack. This helps set the crumb and keeps it moist.
Rest if you can: For the best texture and flavor, wrap the cooled loaf and rest 4–6 hours or overnight before slicing.