Prepare a 8" square glass dish with cooking spray; set aside. In a mixing bowl, sift together sugar, flour, cocoa powder, and salt. Add walnuts and mix well; set aside. In a smaller bowl, combine margarine, egg whites, and vanilla extract. Add to dry ingredients. Mix until thoroughly blended. Spread batter evenly into prepared dish. Microwave, uncovered, on medium-high for ten minutes, or until toothpick inserted in center comes out clean. Rotate dish every two minutes while cooking. When brownies have cooled, spread with frosting.