Preheat the oven to 425°F and prepare a 12-cup muffin pan with liners or nonstick spray so the muffins release easily after baking.
Mash the ripe bananas in a bowl using a fork until smooth with no large lumps, ensuring even flavor throughout the batter.
In a separate bowl, whisk together the flour, baking soda, baking powder, cinnamon, and salt until evenly combined and set aside.
Beat the butter in a large bowl until creamy, then add the brown sugar and mix until the texture becomes light and fluffy.
Add the mashed bananas, eggs, buttermilk, and vanilla extract, mixing until everything is fully combined and smooth.
Gradually add the dry ingredients to the wet mixture and stir gently just until combined, being careful not to overmix the batter.
Fold in 1/2 cup of the chopped walnuts, distributing them evenly without deflating the batter.
Divide the batter evenly among the muffin cups, then sprinkle the remaining walnuts over the tops for added texture.
Bake at 425°F for a few minutes, then reduce the oven temperature to 350°F and continue baking until a toothpick inserted comes out clean.
Let the muffins cool in the pan briefly before transferring them to a wire rack to cool completely.