Preheat oven to 375 degrees F. Coat a standard size muffin pan with nonstick spray.
In a medium bowl, stir together the flour, cocoa powder, sugar, cinnamon, baking powder, baking soda, and salt.
Grate the zucchini with a box grater and squeeze as much liquid out as you can. Measure out 1 cup.
To a medium bowl, add the zucchini, milk, eggs, butter, and vanilla. Whisk to combine.
Gently stir in the flour mixture. Gently stir in the chocolate chips (or blueberries).
Divide batter among the prepared muffin pan, using a heaping 1/4 cup in each. Bake for 16-18 minutes, or until a cake tester inserted into the center of each muffin comes out clean.
Let cool for a minute or two in the pan, then transfer to a wire rack to cool to room temperature.
Serve muffins slightly warmed or at room temperature.