Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan or spray with a non-stick baking spray.
Step 2: In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, salt, and cinnamon until well combined.
Step 3: In a separate medium-sized bowl, whisk together the eggs, vegetable oil, vanilla extract, and milk until smooth.
Step 4: Pour the wet ingredients into the dry ingredients. Mix on low speed with an electric mixer or by hand with a spatula until just combined. Do not overmix; a few lumps are fine.
Step 5: Gently fold in the grated zucchini. The batter will be thick, and it will seem like a lot of zucchini, but it will bake down beautifully.
Step 6: Pour the batter evenly into the prepared baking pan. Spread it out with a spatula if necessary.
Step 7: Bake for 35-45 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. Baking times may vary depending on your oven.
Step 8: Let the cake cool in the pan on a wire rack for 15-20 minutes before inverting it onto the rack to cool completely. Once cool, frost as desired or simply dust with powdered sugar before serving.