This frosting is fluffy and smooth and perfect. It starts by boiling milk and flour together to make a paste and that is whipped into butter to make a frosting that isn't super sweet, but is really light and airy. It is a lot like whipped cream, but more stable. It is a classic topping for red velvet cake, but is great in a variety of applications!
This first step is the most crucial part to this frosting turning out. In a saucepan, whisk together the flour and sugar. Add the milk a few tablespoons at a time to get started. Stir until there are no lumps. Once you have a smooth paste, add the remaining milk and stir until combined. Turn on the burner to medium heat. Continue to whisk and cook until the mixture thickens to a paste consistency. You want it to be pretty thick. It will be tempting to walk away and so something else, but don't. Once it starts to thicken it will go really quickly and it is important you don't get lumps or scorch the bottom. Place paste in the refrigerator to cool.
Once paste is room temperature or cooler, add butter and vanilla to mixer bowl. Turn on low and stir until smooth. Add flour/milk paste and whip on high for about 10 minutes. Don't try to cut the time short, you really want to whip a lot of air into it.
Spread on cakes, cupcakes, cookies or whatever scrumptious treat you are making!
Notes
This recipe makes about 3 cups of frosting, which is plenty for a 9x13-inch cake. It can also frost 12-18 cupcakes depending on how generous you are with the frosting. It will cover an 8 or 9 inch layer cake with a thing coating of frosting, but you may want to make a little bit extra if you want extra to decorate it.