Preheat oven to 350 degree F. Grease and flour a 9x9x2-inch baking pan; set aside.
In a medium bowl combine 1-1/2 cups of the cake mix, sugar, and cocoa powder. Cut in butter until crumbly; stir in walnuts and chocolate pieces. Press onto bottom of pan. Bake 15 minutes. Cool 15 minutes.
Combine remaining cake mix, oil, 2/3 cup water, and eggs; beat 2 minutes. Pour onto crust. Bake 35 minutes or until a toothpick inserted near center comes out clean. Cool on a wire rack. Top with Chocolate Icing. Makes 12 servings.
In a small saucepan combine semisweet chocolate pieces and light cream or milk; heat and stir until smooth.