Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch (23x13 cm) loaf pan, or line with parchment paper, leaving an overhang on the long sides for easy removal.
Step 2: In a large bowl, thoroughly mash the very ripe bananas with a fork or potato masher until mostly smooth with a few small lumps remaining.
Step 3: Add the melted butter to the mashed bananas and stir until well combined.
Step 4: Stir in the granulated sugar and packed light brown sugar until fully incorporated.
Step 5: Beat in the eggs, one at a time, until each is fully combined into the mixture.
Step 6: Stir in the sour cream (or Greek yogurt) and vanilla extract until smooth.
Step 7: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt until thoroughly combined.
Step 8: Gradually add the dry ingredients to the wet ingredients, mixing with a spoon or spatula only until just combined. Be careful not to overmix; a few streaks of flour are acceptable.
Step 9: If using optional add-ins like chocolate chips or nuts, gently fold them into the batter.
Step 10: Pour the batter into your prepared loaf pan and spread it evenly.
Step 11: Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean or with a few moist crumbs attached. If the top browns too quickly, loosely tent with aluminum foil.
Step 12: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes.
Step 13: Carefully invert the loaf onto a wire rack and let it cool completely before slicing and serving. This ensures the perfect texture and prevents sogginess.