Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line it with parchment paper for easy removal.
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. Set aside.
Step 3: In a separate medium-sized bowl, cream together the softened unsalted butter and both sugars (brown and granulated) using an electric mixer until light and fluffy, about 2-3 minutes.
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: Fold in the mashed bananas into the wet ingredients. Don't worry if the mixture looks a little separated at this point.
Step 6: In another bowl, whisk together the all-purpose flour, baking soda, salt, and any desired spices like cinnamon or nutmeg until well combined.
Step 7: Gradually add the dry ingredients to the wet ingredients, mixing until just combined with a spatula. Do not overmix; a few streaks of flour are fine.
Step 8: Stir in the buttermilk until just incorporated.
Step 9: Pour the batter into your prepared loaf pan, spreading it evenly. Optionally, sprinkle extra brown sugar or chopped nuts on top.
Step 10: Bake for 55-70 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. If the top browns too quickly, tent loosely with aluminum foil for the last 15-20 minutes.
Step 11: Let the banana bread cool in the pan for 10-15 minutes before carefully inverting it onto a wire rack to cool completely.