Preheat oven to 350ºF. Line muffin tin cups with liners and coat with cooking spray.
Whisk together banana, light butter, brown sugar, egg, vanilla, and milk.
In a separate bowl, whisk together flour, cocoa powder, baking powder, baking soda, cinnamon, and salt.
Gently fold flour mixture into banana mixture, being sure not to overmix.
Fill muffin cups half way with batter. Sprinkle about half of the chocolate chips evenly among the cups, then continue to evenly disperse remaining batter among the cups.
Sprinkle remaining chocolate chips evenly across the top of the muffins. Lightly push them into the batter.
Bake for 20-25 minutes, or until a toothpick inserted into the center come out clean.