Miracle Frosting

In a bowl whisk sugar, flour, cornstarch, and salt, slowly whisk in milk. Strain mixture through a strainer and into a medium-sized saucepan set over medium heat. Whisk continuously until the mixture thickens, about 8 to 10 minutes.
Transfer to a bowl, cover in plastic wrap, and let it cool to room temperature, about 2 hours.
With a mixer whip the milk mixture until softened up, about 30 seconds. Remove half of the whipped milk-mixture and gradually add in the butter and shortening a few pieces at a time.
Once butter and shortening are all added, add the removed milk-mixture back into the bowl and continue to whip the frosting for another 5 minutes. Use immediately to decorate and enjoy!