Mix sugar and flour in a large bowl.
Melt margarine, cocoa and water in saucepan. Bring just to a boil and add to mixture in bowl. Mix baking soda into buttermilk. Add to flour mixture with remaining ingredients and blend.
Pour batter into lightly greased 15-1/2x10-1/2x1-inch jelly roll pan and bake at 400 degrees for 20 minutes.
While cake is baking, prepare the frosting, which is a major part of the presentation.
Frosting: Melt margarine, milk and cocoa in saucepan. Remove from heat and blend in sugar and vanilla. Mix in nuts and spread on cake while cake and frosting are still warm. Cool; cut into squares.
Yield: About 15 squares (3x3-1/2-inches).