In the bowl of a mixer, cream together the shortening, eggs and sugar. In a separate small bowl, mix together the cocoa and food coloring. Add the paste to the shortening mixture. Sift the flour and salt together over parchment. Add to the batter alternately with the buttermilk in 3 additions. Add the vanilla extract. Fold in the baking soda and vinegar. Pour the batter into 2 greased 9-inch cake pans. Bake for 30 minutes or until an inserted cake tester comes out clean. Let cool on a cooling rack. Invert the cakes from the pans.
Frosting: Over medium heat, cook the flour and milk until thickened. Let cool. In the bowl of an electric mixer, cream together the butter, sugar and vanilla until light and fluffy. Beat in the flour mixture.
Frost the top of the first layer with frosting and set the second layer on top. Frost the entire cake with remaining frosting.