In a large saucepan, whisk together flour and 1/2 cup of milk until smooth. Set over medium heat and and whisk in remaining milk and salt. Whisk constantly until mixture is thick and paste-like - AND SMOOTH. Remove from heat and let cool completely.
In the bowl of a stand mixer fitted with the whisk attachment, combine butter and sugar. Beat until light and fluffy. Add vanilla and mix well.
Beat cooled flour mixture into the butter mixture on high speed. Beat for several minutes (6-10) until sugar granules have dissolved and the mixture is super silky and fluffy. You can check this by rubbing a little between your thumb and forefinger.