Preheat the oven to 375ºF (190ºC) and line a 12-cup muffin tin with muffin cups.
In a large bowl, whisk together the flour, cocoa, sugar, baking powder, baking soda, and salt.
Add the milk, oil, vanilla, and zucchini to the flour mixture. Stir until just combined and no streaks of flour remain. Fold in the chocolate chips.
Scoop equal amounts of batter into the muffin tins, a little over three-quarters full, until all of the batter is used. Top each muffin with a few extra chocolate chips.
Place the tin in the pre-heated oven and bake for 20-22 minutes, or until a toothpick comes out clean when inserted.
Place the baked muffins onto a wire rack and cool in the tins for 10 minutes before removing and cooling fully. The muffins can be stored in a sealed container at room temperature for 3-4 days, or frozen.