In a mixing bowl, combine oil, sugar, egg and vanilla until well mixed. Blend in the zucchini. Combine the flour, baking powder, baking soda and salt; add to zucchini mixture and mix well.
Divide batter in half. Add cocoa to half; spread into a greased 13x9x2-in. baking pan. Pour remaining batter on top and spread out evenly.
Bake at 350 degrees for 25 minutes or until bars test done. Cool.