Procedure
Take a pan, add flour with milk and heat the mixture till everything is incorporated into a thick paste. Whisk in the sugar, butter, and shortening in a different bowl to get a smooth paste. Turn the heat off and set the pan aside to let it cool. Once the mixture is cool, add it to the sugar paste. Beat everything together till you get the perfect whip consistency. Prepare it couple of hours before you actually have to use it.