In a small saucepan, whisk flour into milk and heat, stirring constantly, until it thickens. It will be the consistency of a thick pancake batter. It should be like a thick paste, but it should still be wet and stir-able.
Remove from heat, and let it cool to room temperature. This step is key. If the milk mixture is warm, it won't come together properly.
Stir in vanilla bean paste.
While the mixture is cooling, cream the butter, sugar, and salt together until light and fluffy on medium high, about 5 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. It may look separated at first, so keep beating it until it comes together and looks like a whipped cream.