Preheat oven to 350 F.Grease and flour 9"x13" pan. In large bowl, with mixer at low speed, beat sugar, margarine or butter, vanilla, and eggs until blended. Increase speed to high, beat until light and fluffy, about 5 mins. Reduce speed to low; add flour, sour cream, cocoa, baking soda and salt; beat until blended, constantly scraping bowl. Increase speed to medium;beat 1 minute, occasionally scraping bowl. With spatula,fold in zucchini, chocolate pieces and nuts. Batter will be thick. Spread batter evenly in pan. Bake 40-45 minutes until toothpick inserted in center of cake comes out clean. Cool cake in pan on wire rack. To serve, sprinkle with confectioners sugar.