Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or line with cupcake liners.

Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg together in a large bowl. Set aside. In a medium bowl, whisk the oil, granulated sugar, brown sugar, eggs, vanilla extract, and milk together. Whisk in the shredded zucchini. Pour the wet ingredients into the dry ingredients and whisk or stir until just combined. Fold in any add-ins you want to include, if any. Avoid over-mixing. Batter is thick.

Spoon the batter into liners, filling them all the way to the top. Sprinkle with coarse sugar, if desired.

Bake for 5 minutes at 425°F (218°C); then, keeping the muffins in the oven, reduce heat to 350°F (177°C) and continue baking for another 15–17 minutes or until a toothpick inserted in the center comes out clean. The total time these muffins take in the oven is about 20–21 minutes, give or take. (For mini muffins, bake 11–13 minutes at 350°F (177°C) the whole time.)

Allow the muffins to cool for 5 minutes in the muffin pan, and then transfer to a wire rack to continue cooling, or enjoy warm.

Muffins stay fresh covered at room temperature for a few days, then transfer to the refrigerator for up to 1 week.