Preheat the Oven and Prepare the Muffin Tin:
Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or lightly grease the cups with cooking spray.
Mix the Dry Ingredients:
In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon (if using). Set aside.
Cream the Butter and Sugar:
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes. You can use an electric mixer or a hand whisk.
Add the Wet Ingredients:
Add the mashed bananas, egg, and vanilla extract to the butter mixture. Mix until well combined.
Add the Dry Ingredients to the Wet Ingredients:
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.
Add Milk and Walnuts:
Stir in the milk and chopped walnuts (or your preferred nuts) until everything is evenly combined. If you prefer a smooth batter, you can leave the nuts out or add them on top after portioning the batter.
Fill the Muffin Tin:
Spoon the batter into the muffin cups, filling each about 2/3 full. You can use an ice cream scoop or a spoon to help with this step.
Bake the Muffins:
Bake the muffins in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool the Muffins:
Once baked, remove the muffins from the oven and let them cool in the tin for a few minutes. Then transfer the muffins to a wire rack to cool completely.