Preheat your oven to 325°F (165°C). Grease a 9x5-inch loaf pan with cooking spray or butter, then line it with a parchment paper sling, leaving overhang on the long sides for easy removal.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon until well combined and free of lumps.

In a large mixing bowl, mash the very ripe bananas with a fork until mostly smooth with a few small lumps. You should have about 1 1/4 to 1 1/2 cups mashed banana.

Add the granulated sugar and brown sugar to the mashed bananas and whisk until blended and glossy. Whisk in the eggs one at a time, then whisk in the oil, buttermilk, and vanilla until the mixture is thick, creamy, and smooth.

Sprinkle the dry ingredient mixture over the wet mixture. Using a spatula or wooden spoon, gently stir just until no dry flour remains. A few small lumps are fine; do not overmix.

Fold 1/2 cup of the chopped walnuts into the batter, reserving the remaining 1/4 cup for topping.

Pour the batter into the prepared loaf pan and smooth the top with a spatula. Sprinkle the remaining 1/4 cup chopped walnuts evenly over the surface.

Bake at 325°F (165°C) for 55–70 minutes, or until the top is deep golden brown and a toothpick inserted into the center comes out mostly clean with a few moist crumbs. If the top browns too quickly, tent the loaf loosely with foil for the last 10–15 minutes.

Place the pan on a cooling rack and let the bread cool in the pan for about 15 minutes. Use the parchment sling to lift the loaf out of the pan, then transfer it to the rack to cool completely, about 45–60 minutes.

Once fully cooled, slice the banana bread into thick slices using a serrated knife. Serve plain, or with butter or cream cheese, and enjoy with coffee, tea, or milk.