Preheat oven to 450 degrees Fahrenheit. Line a muffin tin with 12 paper liners. Lightly grease paper liners with nonstick cooking spray.
In a large mixing bowl, use a hand mixer to cream together bananas, butter, and sugar, about two minutes.
Beat in eggs until combined. Add in the baking powder, salt, cinnamon (if using), and vanilla and mix briefly.
Add in half of flour, mix with hand mixer until just combined, then add in the milk, stirring to combine. Scrape the bottom and sides of the bowl and add in the remaining flour until just combined. Fold in nuts or mini chocolate chips (if using).
Divide the batter up equally amongst the 12 muffins, filling almost to the top of the muffin tin.
Bake in the preheated oven for 5 minutes. Reduce the oven temperature to 350 degrees and continue baking another 14 to 16 minutes, until a toothpick inserted into the center comes out clean.
Let muffins cool for 5 minutes in the pan before removing them and transferring to a wire rack to cool completely.