Whisk flour into milk and place over medium heat in a small sauce pan.
Cook mixture until thickened; remove from heat and add salt;
Cover with plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate about an hour to cool.
In a stand mixer, begin creaming the butter and sugar until fluffy.
Add vanilla. When milk mixture is slightly cooled; add 1 TB at a time to the creamed butter mixture while the mixer is running on medium speed.
Slowly but surely the gluten will pull in the butter into an emulsion.
You should have an extremely fluffy, light and buttery frosting when completed. Don't be alarmed if it looks a little curdled. The original recipe said that it is perfect when it looks this way. Enjoy!