Preheat oven to 350°F. Spray a 13 x 9-inch baking pan with cooking spray.
Combine flour, cocoa, baking powder, soda, salt, cinnamon and cloves in a medium bowl; set aside.
Beat oil, butter and sugar until smooth in a large bowl with mixer at medium speed. Add eggs, buttermilk and vanilla; blend well. Blend in flour mixture; fold in zucchini and walnuts, if desired.
Spread batter in prepared pan; sprinkle chocolate chips evenly over top.
Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool on wire rack. Delicious served with a scoop of vanilla ice cream.