In a small saucepan, whisk flour into milk and heat, stirring constantly, until it thickens. It will be the consistency of a thick paste.
7 tablespoons all-purpose flour, 1 ¼ cups milk
Remove from heat and let it cool to room temperature.
Stir in vanilla.
1 ½ tablespoons pure vanilla extract
While the mixture is cooling, cream the butter, sugar, and salt together until light and fluffy on medium high, about 3 minutes.
1 ½ cups salted butter, 1 ½ cups brown sugar, Pinch of salt
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.
Video
Notes
Butter temperature: Make sure that your butter isn’t too warm! The butter should be at cool room temperature. You want it to dent if you press it, but you don’t want it to be so warm that it’s near melting.
Milk/flour mixture: When cooking the milk/flour mixture, you don’t want it to look wet. It should be like a thick paste. If it’s wet and runny, your frosting will be too loose.
Mixing: If you’re using a hand mixer, beat it on high. This recipe work well with a stand mixer because it has the power to really whip the frosting.
Mixing time: Please don’t cut the mixing time short! I know it’s tempting, but it’s important to beat the butter and sugar together for the time the recipe calls for to keep it from being gritty. If you cut the mixing times short, you won’t end up with the right texture of frosting.
To get the best flavor, you’ll want to use real butter and pure vanilla extract.