Preheat oven to 350 degrees F.
In the bowl of a standing mixer, combine oil, sugar and vanilla.
In a separate bowl, whisk together flour, cocoa powder, baking soda and salt.
In 3 batches, add flour mixture to oil/sugar mixture beating well between batches and scraping sides of the bowl.
(Batter will be crumbly at this point but will moisten when zucchini is added.)
With mixer on the lowest speed, add zucchini and mix well.
Pour batter into prepared 9 x 13 pan.
Bake 30 minutes. Knife inserted in center should come out clean.
Cool. Dust with powdered sugar before serving if desired.