Begin by preheating your oven to 425°F (218°C). Prepare a 12-count muffin tin by coating it with nonstick spray or lining it with cupcake liners.
In a large mixing bowl, combine the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set this mixture aside. In a separate medium bowl, whisk together the oil, granulated sugar, brown sugar, eggs, vanilla extract, and milk. Once blended, stir in the shredded zucchini. Gradually add the wet mixture to the dry ingredients, stirring just until combined. Fold in any optional add-ins at this stage. Be careful not to over-mix the batter, which will be thick.
Divide the batter evenly among the prepared muffin liners, filling them to the top. If desired, sprinkle coarse sugar over the tops for a bit of extra crunch.
Place the muffin pan in the oven and bake for 5 minutes at 425°F (218°C). Without removing the muffins, reduce the oven temperature to 350°F (177°C) and continue baking for an additional 15–17 minutes. The muffins are done when a toothpick inserted into the center comes out clean, with the total baking time around 20–21 minutes. (For mini muffins, bake at 350°F (177°C) for 11–13 minutes.)
Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to finish cooling. These muffins can be enjoyed warm or at room temperature.
Store the muffins in an airtight container at room temperature for a few days. For longer storage, place them in the refrigerator, where they will stay fresh for up to one week.