Preheat oven to 350° F and spray muffin tin with nonstick cooking spray or line with paper liners.
In a medium bowl, or the bowl of a stand mixer, mash bananas until they are mostly smooth. Beat in the brown sugar, egg, vanilla extract, and peanut butter. Mix in the milk.
Add flour, baking powder, baking soda, salt, and cinnamon and mix until combined.
Scoop out batter into muffin tin, filling each 2/3 full. Bake at 350° F for 18-20 minutes or until toothpick inserted in center comes out clean.
Remove from muffin tin and place on wire rack to cool completely.