Preheat oven and whisk dry ingredients: Preheat oven to 350°F. Line a standard 12-cup muffin tin with baking cups. In a medium bowl, whisk together flour, baking powder, salt, and cinnamon.
Combine banana, sugar, egg, butter, and milk: In a separate bowl, whisk together banana, sugar, egg, butter, and milk.
Mix wet ingredients into dry: Stir wet ingredients into dry ingredients.
Add chocolate: Stir in chocolate chips.
Divide batter and bake muffins: Divide batter evenly between prepared cups, adding a scant ¼ cup to each. Bake until tops spring back when lightly touched, about 20 minutes.
Cool: Cool in pans 5 minutes, then remove to wire rack to cool completely.
