Heat oven to 400 deg. Grease and flour 15x10-inch jellyroll pan. In large bowl,combine sugar,flour,cocoa,soda and salt. In small saucepan,bring butter and water to a boil;stir into sugar mixture. Whisk in buttermilk,eggs and vanilla until throughly blended. Pour batter into prepared pan (batter will be thin) Bake 20 minutes or until toothpick inserted in center comes out clean. Frost while warm. Let cool,then cut into 24 pieces.
In a large bowl,combine sugar an walnuts. In small saucepan,bring butter,cocoa and milk to a boil. Stir in sugar mixture;blend throughly. Spread on warm cake.