Preheat oven to 350°F (180°C).
Combine flour, cocoa, baking powder, baking soda and salt; set aside.
Cream together butter and sugar. Beat in eggs, one at a time. Beat in vanilla, orange rind and zucchini. Stir in walnuts.
Add dry ingredients to creamed mixture and stir until ingredients are just blended. Turn batter into two greased 8″ (20 cm) round layer pans.
Bake for 20 – 25 minutes or until top springs back. Remove from pans and cool on wire racks. Frost.
Turn batter into a greased 13″ x 9″ (3.5 L) cake pan. Bake for 40 – 45 minutes or until top springs back. Cool completely and frost.
FROSTING: Beat together the butter, egg and orange rind. Add cocoa and icing sugar and beat. Gradually add milk and beat until frosting reaches spreadable consistency.