Preheat the oven to 350ºF. Line a muffin pan with paper liners or spray with non-stick cooking spray.
In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
In a large bowl, whisk egg and brown sugar until smooth and no lumps remain. Add vanilla extract, butter, and milk. Whisk again until smooth, then use a rubber spatula or wooden spoon to stir in zucchini and mashed banana. Add the dry ingredients a little at a time, folding the batter until just combined. Fold in the chocolate chips. Divide batter amongst muffin cups.
Bake for 15-17 minutes or until a toothpick inserted in the center of a muffin comes out clean. Remove to a wire rack to cool completely. Top with peanut butter nutella glaze if desired.
To make the glaze:
In a small bowl, combine sugar, peanut butter, and nutella. Add milk a few drops at a time, mixing well until desired consistency is reached.