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 with your hands. (I usually place it onto a clean kitchen towel and roll up to remove excess moisture, too.) Measure out 1 cup after this process.
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.
Notes
Store in an airtight container at room temperature for 3 days or in the fridge for up to 5. Freeze cooled muffins in a zip top freezer bag with as much air removed as possible for up to 3 months.
Serve at room temperature, warmed slightly, or chilled. (They may firm up a bit if chilled.)
If using blueberries, expect the muffins to be super moist.
Use 1/2 cup sugar if you prefer your muffins to be sweeter. These are very lightly sweetened as written.
Mini Muffins: Cook for 12-14 minutes in a 24-cup mini muffin pan.
Dairy-free: Use melted coconut oil in place of the butter and plain nondairy milk.
Gluten-free: Use cup-for-cup style gluten-free flour in place of the whole wheat.
Egg-free: Use a purchased egg replacer or omit the eggs, increase the milk to 1 1/2 cups, and increase the baking soda to 1 teaspoon. Bake for 18-22 minutes and check for doneness with a cake tester. (I tested them and it worked well, though they are moister than the regular recipe so they take a little longer to bake.) Let cool fully before eating for the best texture.
Use the same amount of honey or maple syrup in place of the sugar.
Use white whole wheat or all purpose flour if desired.