Preheat oven to 350° F. Grease and flour a 13x9-inch baking pan.
Combine sifted flour, baking powder, salt and cocoa in bowl.
Cream oil and sugar in separate bowl. Add eggs one at a time, beat well after each addition. Add food coloring, mix until well blended.
Combine buttermilk and soda. Mix well. Add to oil-sugar mixture alternately with dry ingredients. Add vinegar and vanilla extract. Mix well and bake 25 to 30 minutes. Frost with Red Velvet Frosting.
Cook flour and milk until thick. Stir constantly. Cool COMPLETELY. Cream margarine and sugar well. Add vanilla extract; bea Tbsp Add flour and milk mixture. Beat until right consistency to spread. Will get really fluffy.
Servings: