Preheat oven to 400° F.
Spray muffin pan with cooking spray.
Combine oat flour, wheat flour, brown sugar, baking soda, and salt in a medium bowl.
In a large bowl, combine egg, oil, banana, water, maple syrup, and vanilla. Whisk to combine. Stir in the zucchini and then mix in the dry ingredients. Stir until just combined then blend in chocolate chips.
Portion into muffin pan. Add walnut to top if desired. Bake for about 10 minutes for mini muffins and 15-18 minutes for regular-sized muffins. Muffins are done when golden on top and a toothpick inserted into the center comes out clean.