Allow eggs to stand at room temperature for 30 minutes. Meanwhile, grease and flour two 9x1-1/2-inch round cake pans or one 13x9x2-inch baking pan. Set pan(s) aside. In a small bowl stir together cocoa powder and food coloring; set aside. In another small bowl stir together flour and salt; set aside.
Preheat oven to 350 degrees F. In a large mixing bowl beat shortening with an electric mixer on medium to high speed for 30 seconds. Add sugar and vanilla to shortening; beat until well combined. Add eggs 1 at a time, beating on medium speed after each addition until combined. Beat in cocoa mixture. Alternately add flour mixture and buttermilk, beating on low to medium speed after each addition just until combined. Stir together baking soda and vinegar. Add to batter, mixing until combined. Pour batter into prepared pan(s).
Bake for 30 to 35 minutes for round pans, about 30 minutes for 13x9x2-inch pan, or until a wooden toothpick inserted near center comes out clean. Cool cake layers in pans for 10 minutes. Remove cake layers from pans; cool thoroughly on wire racks. Or, place the 13x9x2-inch cake in pan on a wire rack; cool thoroughly. Frost with Creamy Frosting. Cover and store in the refrigerator for up to 3 days. Makes 12 servings.
In medium saucepan whisk milk into flour. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir for 2 minutes more. Cover surface with plastic wrap. Cool to room temperature (do not stir). In medium mixing bowl beat softened butter, sugar, and vanilla with electric mixer on medium speed until light and fluffy. Add cooled milk mixture to butter mixture, 1/4 cup at a time, beating on low speed after each addition until smooth.