Cooked Milk Frosting

In a small saucepan, whisk together flour and milk until smooth. Cook over medium-high heat, whisking constantly, until mixture becomes thick and resembles pudding, 3 to 4 minutes. Transfer to a heat-proof bowl; stir in vanilla and salt. Press plastic wrap against surface; let cool completely.

Beat butter with sugar on medium speed until light and fluffy, scraping down bowl as needed, about 2 minutes. Add milk mixture and continue beating until frosting is light and fluffy and resembles whipped cream, about 2 minutes more. Use immediately.