Preheat oven to 350°F (175°C). Line a muffin tin with paper liners.
In a large bowl, mix together the zucchini, oil, sugar, eggs, and vanilla.
In another bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and cinnamon.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips.
Divide the batter evenly among the muffin cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.