1Whisk flour, sugar, and salt together in a saucepan over medium heat.
2Slowly add the milk to the saucepan while stirring well to ensure no clumps remain.
3Stir consistently once it begins to bubble to prevent scorching, until the bottom of the pan is visible while stirring.
4Scrape the mixture into a bowl and cover with plastic wrap pressed directly against the surface of the frosting base, then refrigerate until completely cool.
5Mix the softened butter in a stand mixer until fluffy, scraping down the bowl as needed.
6Slowly add the cooled flour and sugar mixture to the whipped butter and whip until smooth, light, and silky.