Preheat oven to 350°F. Grease and flour two 9-inch cake pans.
Combine the sifted flour and salt, and set aside.
Put the cocoa in a small glass bowl, and add the food coloring gradually, stirring until mixture is smooth. Set aside.
Cream together the shortening and sugar, beating for 4 or 5 minutes at medium speed in your electric mixer until fluffy. Add the eggs, one at a time, beating for at least 30 seconds after each addition.
At low speed of your mixer, add the flour mixture to the sugar mixture alternately with the buttermilk and vanilla, scraping down the sides of the bowl as necessary. Add the cocoa/food coloring mixture, mixing until color of batter is uniform. Do not overbeat; overbeaten cake batter will result in a tough cake. Turn off your mixer.
In a small bowl, mix the vinegar with the baking soda. It will foam up. Stir it briefly to mix, and then add it to the cake batter, folding it in to incorporate well, but do not beat.
Pour the batter into the prepared cake pans, and bake in a 350°F oven for 25 to 30 minutes, or until a cake tester comes out clean. Allow layers to cool on a rack for 10 minutes before turning out. Let cake cool completely before frosting.
Don't be tempted to shorten the shortening/sugar creaming time. Cake texture will be far better, and you can mix up your cocoa and food coloring while the mixer is running.
Mixing the cocoa with the food coloring is the best way to get uniform color in the cake. On my first test cake, I sifted the cocoa together with the flour, twice, and still came up with little cocoa-colored whorls in the finished cake.
Yes, red food coloring does stain, so use glass utensils rather than plastic. And be careful.
The recipes I found on the Internet came up about fifty/fifty as far as the amount of red food coloring they called for -- some with one ounce, the rest with two ounces. Let me just say this: The cake with one ounce of food coloring was a very dark pink. The cake with two ounces (1/4 cup, or 4 tablespoons) was RED.
The recipes also varied on the amount of vinegar with some calling for as much as one tablespoon. I found that one teaspoon works just fine. And white vinegar with its less pungent odor is definitely preferable to apple cider vinegar.
Most recipes stated that this recipe was suitable for three 8-inch layers and one 9x13-inch cake, but I did not find it to be necessarily so. If you want more than two layers, better to bake two 9-inch layers and split them. A 9x13-inch cake will work, but requires careful testing by the cook after 25 minutes.
Beautiful Thick White Frosting
1-1/2 cup sugar
1/2 teaspoon cream of tartar
1/8 teaspoon salt
1/2 cup water
4 egg whites (at room temperature)
Combine sugar, cream of tartar, salt and water in heavy saucepan. Cook over medium heat, stirring constantly, until mixture is clear. Cook until mixture reaches 240°F on a candy thermometer (soft ball stage).
Beat egg whites until soft peaks form. Let mixer continue to run and slowly pour the sugar mixture in a thin stream down the side of the mixer bowl (don't let the sugar mixture come into contact with the beaters). Continue beating until stiff peaks form and frosting thickens to desired consistency.
I admit that this frosting recipe is not the one most often paired with Red Velvet Cake. The traditional recipe begins by cooking a mixture of flour and milk, cooling it, and then whipping in butter, sugar and vanilla. But the resulting frosting, however good it might be, must be refrigerated, and I don't like my cake cold. The above frosting recipe will yield an extravagant amount of lavishly beautiful and delicious white frosting.