Preheat the oven to 350°F (175°C). Line a muffin tin with muffin liners or grease it with non-stick spray.
In a large mixing bowl, whisk together the flour, cocoa powder, granulated sugar, brown sugar, baking soda, baking powder, and salt until well combined.
In another bowl, whisk together the vegetable oil, eggs, and vanilla extract until smooth.
Add the grated zucchini to the wet ingredients, mixing well.
Pour the wet mixture into the dry ingredients and stir gently using a rubber spatula until just combined. Be careful not to overmix; it's okay if there are a few lumps.
Fold in chocolate chips if using, ensuring they are evenly distributed in the batter.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.