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.
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.