Preheat oven to 350ºF. Grease and lightly flour two 9” round cake pans.
Cake: In a small bowl mix together the food coloring and cocoa powder. Set aside. In a large bowl with an electric beater, cream together the shortening and the sugar. Add eggs and the coloring mixture. Beat for 10 minutes. Add buttermilk. Slowly add flour. Add the salt and vanilla. In a measuring cup mix the vinegar and the baking soda; fold into the batter. Mixing lightly. Dividing evenly, spoon batter into prepared pans. Bake for 30-35 minutes or until cake springs back when lightly touched. Set on wire racks to cool.
Frosting: with an electric mixer in a large bowl, cream together butter, shortening and sugar. Add the milk and vanilla, mixing well. Add the flour, a tablespoon at a time. Beat for 12 minutes.
Lay first cake top side down on plate and frost with ½ of frosting. Lay the next cake top side up and cut top to make an even layer. Frost sides and top with remaining icing.