Sift together flour, salt, instant cocoa mix. Cream butter and sugar. When well creamed beat in whole eggs one at a time. Blend well, add food coloring and vanilla. Mix buttermilk, vinegar and soda. Add alternately with dry ingredients to the creamed mixture. Blend at low speed between additions. Put in prepared pans. Bake at 350 degrees F for 30 minutes. Don't over-bake. For Icing: Blend flour and milk. Cook until thickens to cream. Cool. Cream sugar, margarine, shortening, salt and vanilla. Add slightly cooked flour mixture and beat until fluffy.