Follow the instructions on the package to bake the cake.
Let it cool and then cut it in half across the middle.
Put the flour, milk and salt in a pan and cook until thick.
Take the pan off the heat and stir in the vanilla.
Let it cool to room temperature and then refrigerate for 30 minutes.
Cream the sugar, shortening, butter, and salt, then add it into the refrigerated mixture.
Beat for 10 minutes or until fluffy.
Spread it over the bottom layer of the cake and add the top layer back on.
Spread the chocolate frosting on top.