Whisk together flour and sugar in a medium saucepan over medium heat. Cook for about 2 minutes to toast the flour. 

Slowly add in milk, whisk to combine and bring the heat to medium-high. Whisk continuously until mixture is thickened and pudding-like. Cover with plastic wrap (directly on pudding mixture) and let cool completely.

Add butter to the bowl of your stand mixer and whisk on high until light and fluffy. Slowly add in the cooled flour mixture one spoon at a time while continuing to whisk. Incorporating slowly insures a smooth buttercream. 

Add in vanilla and salt until everything is creamy.