Make the icing five minutes before taking the cake out of the oven.
Sift together sugar and next 4 ingredients in a large bowl.
Stir together butter and next 3 ingredients in a saucepan over medium-low heat, stirring constantly, 5 minutes or just until butter and shortening melt. Remove from heat; pour over sugar mixture, stirring until dissolved. Cool slightly.
Stir in buttermilk, eggs, and vanilla. Pour into a greased and lightly floured 15- x 10-inch jelly-roll pan.
Bake at 350° for 30 to 35 minutes. (Cake will have a fudge-like texture.) Spread Chocolate Icing over hot cake. Serve with vanilla ice cream, if desired.