Preheat your oven to 375 degrees F. Coat a 12-cup standard-size muffin tin with nonstick spray. Set aside.
Spread the zucchini out on a few layers of kitchen or paper towels. Press out as much water as possible, changing out the towels as needed. Set aside.
Place the avocado in the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl. Beat with the mixer until the avocado is very smooth and no chunks remain. You should have about 2/3 cups of smooth avocado paste. Add the coconut oil, egg, coconut sugar, milk, and vanilla extract, beating until well combined.
In a separate bowl, stir together the white whole wheat flour, cocoa powder, baking soda, cinnamon, and salt. Gradually add the dry ingredients to the wet ingredients, folding by hand with a wooden spoon or spatula and stirring just until the flour disappears. The batter will be very thick, almost like brownie batter, and the dry ingredients will take several strokes to work in between additions.
Fold in the zucchini and chocolate chips, reserving a few of the chocolate chips to sprinkle on top of the muffins.
Scoop the batter into the prepared muffin cups, filling no more than three-quarters of the way to the top. Sprinkle with the remaining chocolate chips.
Bake for 15 to 18 minutes, until the tops are set and a toothpick inserted in the center comes out mostly clean with just a few moist crumbs clinging to it. Let cool in the pan for 2 minutes, then transfer to a wire rack to cool completely.
TO STORE: Leftovers may be stored at room temperature according to my guide for How to Store Muffins for up to 5 days.
TO FREEZE: You may keep chocolate zucchini muffins frozen in the freezer for up to 3 months.
TIP: If you would like to reduce the calorie count of the muffins, omit or reduce the number of chocolate chips.