Preheat the oven to 350°F (175°C) and line a muffin tin with 12 liners.
In a large bowl, whisk together the flour, baking powder, salt, and cinnamon.
In a separate bowl, combine the mashed bananas, melted butter, brown sugar, egg, vanilla, and milk.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Fold in the chocolate chips or chunks, if using.
Divide the batter evenly among the muffin liners. Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean.