Preheat the oven to 350 degrees F. Spray a 9x13 inch baking sheet or dish with nonstick baking spray.
Grate the zucchini, place it on a towel and sprinkle it with a pinch of salt. Let it sit for 10 to 15 minutes while you make the rest of the cake.
In a large bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda, cinnamon and salt.
In a small bowl, whisk together the eggs, oil and vanilla extract. Whisk the wet ingredients into the dry until combined and smooth. Fold in the zucchini.
Spread the batter in the greased pan. Bake for 35 to 40 minutes, or until set in the center. Let cool completely before frosting. If you’d like to remove the cake from the pan before frosting, let it cool for 30 minutes in the pan then gently invert.