Combine sugar, flour, and salt in a medium-sized saucepan and whisk well to combine and remove any lumps from the flour.

Turn stovetop heat to medium and whisk in milk until mixture is smooth.

Continue whisking constantly until mixture is thickened to a pudding-like consistency (do not crank up the heat or you’ll burn it and the sugar won’t dissolve properly) and the whisk leaves a trail.

Remove from heat and pour into a heat-proof container and cover the surface with a piece of plastic wrap, wax paper, or parchment paper pressed directly against the surface (to prevent a skin from forming). Allow to cool completely to room temperature (alternatively you may make this mixture a day or two in advance, store in the refrigerator, then bring to room temperature before proceeding).

Once flour mixture has cooled, use an electric mixer or stand mixer to beat butter on high speed until it is light and fluffy (this usually takes several minutes, and you may need to scrape the bowl with a spatula).

Reduce mixer speed to medium and gradually add flour mixture, one heaping spoonful at a time, waiting until each spoonful is incorporated before adding the next and beating until all ingredients are well-combined. Scrape sides and bottom of bowl with a spatula, then stir in vanilla extract.

Gradually increase speed to medium-high and whip frosting until it is smooth, light, and airy (it should not seem greasy). If you have any difficulty or experience your frosting splitting please see the troubleshooting section in the blog post.

Use your spatula to deflate the frosting to work out any air bubbles, then use as desired.