Spray a 9x13" baking pan with non stick cooking spray. Preheat oven to 350°.
Whisk sugar, oil, vanilla, and zucchini in a large bowl till blended.
Add flour, cocoa powder, baking soda, and salt. Stir till blended, and fold in nuts if desired.
Spread batter into prepared pan.
Bake at 350° for 30-35 minutes or till toothpick comes out clean. Frost with chocolate frosting while warm, or cool and frost.
Frosting: Stir together butter and cocoa till smooth. Beat in remaining ingredients, adding milk till it is the consistency you like.
Notes
If you have zucchini haters, peel the zucchini before you add it. No one will even know it's there!
Make sure you mix the sugar with the oil. I made the mistake of adding it with the dry ingredients. The dough was VERY thick. (They still tasted great though.)