Preheat the oven to 425°F and line a 12-cup muffin pan with liners or grease it.
In a bowl, mash the bananas with a fork until smooth with no large lumps.
In another bowl, whisk together flour, baking soda, baking powder, cinnamon, and salt. Set aside.
In a large bowl or stand mixer, beat the butter until creamy, about 3 minutes.
Add the brown sugar and mix until light and fluffy, scraping down the sides as needed.
Mix in the mashed bananas, eggs, buttermilk, and vanilla extract until well combined.
Add the dry ingredients to the wet mixture and stir gently until just combined. Do not overmix.
Fold in ½ cup of the chopped walnuts.
Divide the batter evenly among the muffin cups.
Sprinkle the remaining walnuts over the tops.
Bake at 425°F for 5 minutes, then reduce the oven temperature to 350°F and continue baking for 15–17 minutes, until a toothpick inserted comes out clean.
Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
