Grate the zucchini, place it in a sieve to drain, remove as much moisture as possible, and set it aside.
Heat the oven to 350°F (180°C) and line a muffin tray with paper cups.
Whisk the brown sugar and canola oil in a large bowl until the sugar has dissolved. Add the eggs one at a time, then whisk in the milk.
Sift the plain flour, cacao powder, baking powder, and baking soda into a separate bowl. Add the dry ingredients to the wet ingredients and stir until combined.
Gently stir in the grated zucchini with a spatula or wooden spoon, then fold in the chopped dark chocolate or chocolate chips.
Divide the batter among the muffin cups and bake for about 20 minutes, or until the tip of a knife comes out clean.
Cool the muffins completely, then store them in an airtight container.