Preheat the oven to 425°F. Line a standard 12-cup muffin pan with paper liners and spray with nonstick cooking spray. Set aside.
Add the flour, baking powder, baking soda, salt, cinnamon, and nutmeg to a medium bowl. Whisk until evenly combined.
1¾ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon fine sea salt, 1 teaspoon ground cinnamon, ¼ teaspoon ground nutmeg
In a separate large mixing bowl, whisk together the eggs, brown sugar, and granulated sugar. Add the oil, sour cream, and vanilla extract and continue whisking until smooth.
2 large eggs, ¾ cup light brown sugar, ¼ cup granulated sugar, ½ cup neutral oil, ¼ cup full-fat sour cream, 2 teaspoons pure vanilla extract
Add the dry ingredients to the bowl with the wet ingredients. Use a spatula to gently fold everything together just until the flour is incorporated. Avoid overmixing.
Fold the shredded zucchini into the batter until it is evenly distributed.
1½ cups finely shredded zucchini
Divide the batter evenly among the 12 prepared muffin cups, filling each one almost to the top. Sprinkle the tops generously with coarse sparkling sugar.
2-3 tablespoons coarse sparkling sugar
Bake at 425°F for 5 minutes.
Without opening the oven door, reduce the temperature to 350°F. Continue baking for 14-17 minutes, or until the muffins are tall and golden and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Let the muffins cool in the pan for about 5 minutes before transferring them to a wire rack to finish cooling.