Preheat the oven to 350°F/175°C and line a 12-cup standard muffin pan with non-stick parchment liners, or oil-sprayed paper liners. (These delicate muffins stick to silicone cups even when greased.)
In a large mixing bowl, whisk together all of the dry ingredients (flour, cocoa powder, baking powder, baking soda and salt). Set aside.
Grate the zucchini and squeeze out some of the moisture. Peel and grate the sweet potato.
To a high-speed blender or food processor, add all of the wet ingredients in the order they appear for easiest blending (yogurt, oil, egg, syrup or honey, eggs, vanilla, zucchini, sweet potato, baby spinach and banana, if using) until smooth, scraping the sides if needed.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not over mix. 6. Divide batter evenly among the muffin cups, about 1/4 cup batter per cup, and top with the chocolate chips, if desired. Bake on the middle rack for 23 to 25 minutes (I bake for 24), until a toothpick test comes out clean. Allow to cool in the pan for 10 minutes before transferring the muffins to a cooling rack to cool completely.