Preheat the oven to 375F (190C or 180C on a forced fan oven). Line a 12-cavity muffin pan with muffin papers.
Using a paper towel, squeeze out any excess liquid from the grated zucchini. It won't feel dry, but it shouldn't be dripping water.
Whisk together the flour, cocoa, baking powder, baking soda and salt in a medium bowl.
In a separate large bowl, whisk together the oil, granulated sugar, brown sugar, eggs, vanilla and milk. Be sure that the eggs are properly incorporated.
Carefully whisk the dry ingredients into the wet ingredients until no lumps remain. The batter will be glossy and sticky. I typically use a rubber spatula to fold it in first, then whisk until smooth.
Fold in the grated zucchini and chocolate chips using a rubber spatula.
Spoon the batter into the prepared muffin pan. Each muffin paper will be about ¾ full, or slightly more.
Bake muffins in the preheated oven for about 20-25 minutes, or until the tops are rounded and an inserted toothpick comes out clean or with a few moist crumbs.
Cool in the pan for about 10-15 minutes, then transfer to a wire rack to continue cooling.