Preheat oven to 350 degrees. Thoroughly spray a large (18 x 12) sheet cake pan with nonstick baking spray.
Sift together the cake flour and salt. Set aside. In a separate bowl, stir together the buttermilk, vanilla, baking sodas and eggs. Add the vinegar and stir. Set aside.
In a separate small bowl, mix together the cocoa and red food coloring. Set aside. Cream together the shortening and sugar until fluffy. Alternately add the flour mixture and the buttermilk/egg mixture until all combined. Pour in the red mixture and beat until combined.
Pour the batter into the prepared cake pan. Use a large spatula or knife to even out the surface. Bake for 20 minutes. Remove from oven and let cool for 20 minutes.
For the Frosting: Combine the milk and flour in a saucepan over medium heat and whisk constantly until thick. Remove from the heat and let cool to room temperature. Add the vanilla once cooled.
In a bowl of a stand mixer, cream the butter and sugar until light and fluffy. Add the cooled milk mixture and beat very well. It may look seperated, which means you have to keep beating until fully combined. It should be fluffy and light, similar to whipped cream.