Preheat the oven to 350°F. Line a 12 muffin tin with paper liners or brush with melted butter or spray with baking spray.
Melt butter in a small saucepan over medium heat; bring to a low simmer and cook, stirring frequently, until butter is browned, fragrant, and foamy, with brown bits in it, 5 -7 minutes. Set aside to cool to room temperature, about 15 minutes.
In a medium bowl, combine flour, baking soda, cinnamon and salt. Set aside until ready to use.
In a large stand mixer or using a hand mixer, whisk the cooled brown butter and sugar and mix for 3 minutes until combined. Add the eggs, one at a time, scraping down the sides of the bowl to make sure everything is evenly combined. Add the milk, vanilla extract and the smashed bananas and mix for 30 seconds. Slowly add the dry ingredients and mix for a few seconds until everything is well mixed. Fold in the chocolate chips/pieces and nuts (if using).
Divide the batter among the prepared muffin cups. Transfer to the oven and bake for 18-20 minutes or until a toothpick inserted into the center comes out clean. Serve warm. Enjoy!
