Mix and bake the cake as directed, using a 9 by 13-inch pan. Cool and turn out from pan.
Using dental floss, slice cake in 2 lengthwise, so you have 2 layers.
In saucepan, combine the flour and milk. Cook and stir until thick. Set aside to cool.
In bowl combine sugar, salt, shortening, softened butter and vanilla. Beat with electric mixer until fluffy, then add the cooled milk mixture and beat until fluffy.
Put half the cake in a 9 by 13-inch plastic keeper. Spread all of filling over that, then top with the other half of the cake. Cover with lid and store 1 or 2 days ahead. Garnish with powdered sugar and mint before serving.