Preheat oven to 325°. In a large bowl, beat butter, sugar and oil until smooth.
Add eggs, one at a time, beating well after each addition. Beat in vanilla.
Combine milk and buttermilk.
Combine flour, cocoa, baking soda, baking powder and salt; add to batter alternately with milk mixture, beating well after each addition. Fold in zucchini.
Transfer to a greased 13x9-in. baking pan.
Sprinkle with chocolate chips.
Bake 45-50 minutes or until a toothpick inserted in center comes out clean. Cool on a wire rack. Dust with confectioners' sugar.