Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan, or line with parchment paper leaving an overhang on the sides.
Step 2: In a large bowl, mash the very ripe bananas with a fork until mostly smooth with a few small lumps remaining. Set aside.
Step 3: In a separate large mixing bowl, whisk together the melted butter, granulated sugar, and light brown sugar until smooth.
Step 4: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 5: Add the mashed bananas to the wet ingredients and mix well to combine.
Step 6: In a medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt.
Step 7: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined, being careful not to overmix. A few streaks of flour are okay.
Step 8: Pour the batter into your prepared loaf pan. You can optionally sprinkle a little extra sugar or a few thin banana slices on top.
Step 9: Bake for 55-70 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean or with a few moist crumbs attached. If the top browns too quickly, tent loosely with aluminum foil for the last 15-20 minutes of baking.
Step 10: Let the banana bread cool in the pan on a wire rack for 10-15 minutes, then carefully remove it from the pan and transfer to the wire rack to cool completely before slicing and serving.