Sift together flour, cocoa, baking soda and salt; set aside.
Cream together butter, oil and sugar in mixing bowl until light and fluffy, using electric mixer at medium speed. Beat in eggs, one at a time, beating well after each addition. Blend in vanilla.
Add dry ingredients alternately with buttermilk to creamed mixture, beating well after each addition. Stir in zucchini. Pour batter into greased 13" x 9" x 2" baking pan. Sprinkle with chocolate pieces and walnuts.
Bake in 325 degree oven 55 minutes or until cake tests done.
Cool in pan on rack. Cut in squares.