Heat the oven to 350ºF. Grease a 12-cup muffin tin and set aside.Make the flax egg by whisking together the flax and water. Set this aside to gel while you prepare the remaining ingredients.Whisk together the dry ingredients (minus vanilla extract if using) in a large mixing bowl.In a separate bowl, beat together banana, pumpkin, milk and syrup. Whisk in flax egg and pour wet ingredients into dry. Stir to thoroughly combine (the batter will be kind of thick, but still stirable and soft). Fold in the zucchini and chocolate chips.Fill each cup ¾ of the way full. Add water to the ones that are unused. Sprinkle the tops of the muffins with additional chocolate chips if desired (it makes them uber chocolatey and pretty!).Bake on the center rack for 23 - 25 minutes until a cake tester inserted into the center comes out clean.Cool in the pan for 5 minutes, then transfer to a wire rack and cool completely. If desired, reheat in the toaster oven or in the microwave and top with nut butter (optional, but highly encouraged).