In a saucepan, whisk the flour into the milk over medium-low heat until it thickens. Stir it constantly, lowering the heat to low if needed. The consistency should be very thick, like mashed potatoes. This step took about 12-15 minutes.
Remove the pan from the heat and set the pan in a bowl of ice for 5-10 minutes to quicken the cooling process. The temperature of the mixture should be at room temperature.
Stir in the vanilla extract.
If you have a food processor, process the white sugar for a minute or so so that the granules become finer.
While the mixture is cooling, cream together the butter and processed sugar using a stand mixer with the whisk attachment until light and fluffy. Do this for 5 minutes until the sugar is completely creamed and there is no graininess left.
Add the cooled flour mixture and beat it until it all combines and looks like whipped cream. This will take about another 3-5 minutes of beating. Keep scraping the sides of the bowl while the mixture is beating together so that everything gets well incorporated. The mixture have the texture of a fluffy whipped cream. Set aside the frosting until you are ready to frost the cake.
FOR THE CAKES:
Preheat the oven to 350 degrees.
Cream together the shortening and sugar.
Add eggs and mix well.
Add the cocoa and red color and mix again.
Add the buttermilk, salt and flour. Mix well.
Lastly, add the vinegar and baking soda and mix until the batter is smooth.
Grease (with shortening) and flour two 8″ round baking pans.
Divide the batter evenly between the two pans.
Bake the cakes for 25-30 minutes or until the center of the cake bounces back when you touch it.
Allow the cakes to cool for 10 minutes, then turn them onto a wire rack to cool completely.
ASSEMBLE THE CAKE
Once the cakes are cool, place one cake on a cake plate. Spread frosting on top of that layer, then place the other cake on top of the frosted cake. Use the remainder of the frosting to frost the top and sides of the cake.