For the cake: Preheat the oven to 350 degrees F. Grease and flour three 9-inch round cake pans.
Sift together the flour, cocoa, baking soda and salt. Mix the granulated sugar and eggs. Add the oil, slowly beating well as the oil is added. Add the flour mixture alternately with the buttermilk, beginning and ending with the flour and mixing well after each addition. Stir in the vanilla and red food coloring. Divide the batter evenly among the pans and bake for 40 minutes, testing for doneness with a toothpick. Cool the layers in the pans for 10 minutes, and then turn out onto racks to finish cooling while you prepare the frosting.
For the frosting: Cream the cream cheese and butter. Beat in the confectioners' sugar until the mixture is smooth. If you want grey frosting, add 3 to 4 drops of black food coloring and 2 drops of yellow and mix until grey. Add the vanilla and nuts, reserving 2 tablespoons of nuts for garnish.
Spread the frosting between the layers, on the sides and on top of the cake.
Leave the sides unfrosted if you want the vibrant red to show.