Heat oven to 350^. Grease and flour(I use Baker's Joy)a 13x9-inch baking pan.
Stir baking soda into buttermilk in medium bowl; set aside. Stir together cocoa and water until smooth; set aside.
Beat butter, shortening and sugar in large bowl until creamy. Add eggs, vanilla and salt; beat well. Add buttermilk mixture alternately with flour to butter mixture, beating until blended. Add cocoa mixture; blend thoroughly. Pour into prepared pan.
Bake 40-45 minutes or until wooden pick inserted in center comes out clean. Cool completely. Serve with eggnog Sauce.
Stir cornstarch and cold water in a saucepan until smooth. Add milk, sugar and egg yolks; beat with a wire whisk until well thickened. Remove from heat; stir in brandy and vanilla. Cool completely. Sprinkle several dashes of ground nutmeg over top.