In a small saucepan, whisk flour into milk and heat, stirring constantly, until it thickens. It will be the consistency of a thick paste.
Remove from heat and let it cool to room temperature.
Stir in vanilla.
While the mixture is cooling, cream the butter, sugar, and salt together until light and fluffy on medium high, about 3 minutes.
Then add the completely cooled milk mixture.
Beat it for about 5 minutes on medium-high to high until it looks like whipped cream. If you can still taste the sugar granules, keep beating it until you can’t.
So, what is "cool room temperature"? A lot of people think that room temperature butter means that the butter is on the verge of melted or is partially melted. That's too warm. Cool room temperature butter means that if you press it with your finger, your finger will leave an indentation in the butter, but the butter should still hold its shape and should still feel cool to the touch.
Check out the tips and tricks section above the recipe for more helpful hints.