- Heat the oven to 375 degrees. Line a standard muffin pan with paper liners or grease with oil spray. 
- Place the grated zucchini and apple in a clean kitchen towel or a few layer of paper towel and squeeze out the excess liquid. Add the zucchini and apple to a large bowl. 
- Add the eggs, applesauce, oil, maple syrup, yogurt, and vanilla then whisk to combine.  
- In a separate bowl, whisk together the baking powder, baking soda, salt, cinnamon, and flour. Dump the dry ingredients into the wet then mix until just combined. 
- Scoop the batter into the muffin tin, filling each hole to the top. 
- If using the topping, mix the sugar and cinnamon together and sprinkle over each muffin. 
- Bake until lightly golden brown and each muffin no longer looks wet on the top, 18 to 22 minutes. 
- Cool then store in the refrigerator for 4 days or wrap each muffin well and freeze for up to 2 months. (Note: if you keep the muffins tightly covered or in the fridge they will lose the crunchiness on the top, but they will still taste delicious.)