Preheat oven to 350 °F.
Sift together flour, sugar and cocoa. Melt the butter in the cola until mixture is boiling. Mix with dry ingredients with a wooden spoon. Add the salt, buttermilk, soda, marshmallows, slightly beaten eggs and vanilla and mix well.
Pour batter into a greased 9x13z2" baking pan. Bake in the preheated oven for 30 to 35 minutes. Cool slightly before frosting with the following frosting.
In a small saucepan stir together the cocoa, butter and cola. Heat to boiling and pour over the sugar. Stir to mix well. Stir in the vanilla and nuts.