Preheat oven to 350°F (175°C) and line a muffin tin with liners.
Grate zucchini and squeeze out excess moisture using a clean towel.
In a bowl, whisk flour, cocoa powder, baking soda, baking powder, salt, and cinnamon.
In another bowl, whisk eggs, sugars, oil, milk, vanilla, and zucchini until smooth.
Add dry ingredients into wet ingredients and mix until just combined. Do not overmix.
Fold in chocolate chips.
Fill muffin cups ¾ full and top with extra chocolate chips if desired.
Bake for 18–22 minutes or until a toothpick comes out with moist crumbs.
Cool for 10 minutes, then transfer to a wire rack.