Position a rack in the center of the oven, and preheat to 350˚F.
Put shortening, sugar, cocoa, food coloring, flour, salt, vanilla, baking soda and vinegar in the bowl of a stand mixer fitted with the paddle attachment. (You can use a hand mixer if you allow the shortening to soften at room temperature before beginning.) Mix to combine, starting at low speed, then raise the speed to low-medium and mix for about 1 minute. Add the eggs one at a time, mixing for 1 minute after each is absorbed into the mixture. Add the buttermilk in two portions, stopping to scrape the sides of the bowl between additions.
Grease two 9-inch cake pans (2 inches deep) with the butter, and flour them.