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