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

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

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

Add in your vanilla and salt until everything is creamy and then you can frost your cooled cake.