Preheat oven to 350°F and line 7-8 muffin tins (or 9, see notes) with paper liners.
In a medium mixing bowl, whisk together melted butter, egg, vanilla extract, and sugars. Add the flour, baking powder, baking soda, salt, and cinnamon and stir until just combined (the dough will seem dry here - don't worry!). Fold in mashed bananas and chocolate chips just until incorporated (you don't want to overmix).
Fill muffin liners almost to the top (see note to fill less). If desired, place several additional chocolate chips on top of the muffins. Bake for 20-22 minutes until edges are golden brown and centers bounce back when lightly touched. Remove from pan and let cool slightly before eating. Enjoy!
Any chocolate chips work in this recipe.Updated 12/2020: I retested the recipe several times with exact banana measurements for consistency (opposed to "2 bananas") and have updated from 7 to 8 muffins per your feedback and these additional tests. The batter will fit 8 muffin tins almost all the way full, creating larger muffins. Alternatively, you can fill the muffins 3/4ths of the way full to make 9 muffins with a more domed shape after baking.