How to Make It
Whisk together milk and flour in small saucepan until smooth. Whisking constantly, cook over low heat for 2 to 3 minutes or until it reaches the consistency of pudding.
Remove from heat, add sugar and salt, and whisk until sugar dissolves.
Scrape into medium-size bowl and press a piece of plastic wrap or waxed paper directly on the surface to prevent a skin from forming. Refrigerate until cooled, about 30 minutes.
Place butter in a large metal bowl and beat with mixer set to medium speed until light and creamy, about 2 minutes.
With mixer running, add cooled flour mixture in heaping spoonsful. Beat in vanilla. Scrape bowl with a rubber spatula.
Switch to whisk attachment if your mixer has one. Beat frosting on medium-high speed until light and fluffy with the consistency of stiffly whipped cream. Use immediately. Baker’s Tip: If loose cake crumbs are mixing into the frosting, cover the cake in a very thin coating of frosting (called a skim coat) and refrigerate for 15 minutes before applying the rest of the frosting.
If making the cake ahead, cover loosely and refrigerate overnight, and then return to room temperature for serving.