Prepare the Base: In a medium saucepan, combine the milk, flour, and a pinch of salt. Over medium heat, whisk continuously until the mixture thickens to a smooth paste, about 5 to 7 minutes. Remove from heat and let cool to room temperature. To prevent a skin from forming on the mixture, cover it directly with plastic wrap.

Cream the Butter and Sugar: In a large mixing bowl, use your electric mixer to beat the butter and sugar until light and fluffy, about 3 to 4 minutes. Ensure that the sugar is fully integrated and the mixture is no longer grainy.

Combine the Mixtures: Gradually add the cooled milk mixture to the creamed butter and sugar, beating on high speed until the frosting is light and looks like whipped cream, which should take about 3 to 5 minutes. Stir in the vanilla extract just until combined.

Frost Your Cake: Use the frosting immediately, or cover and refrigerate until needed. It spreads beautifully and works well for creating decorative designs.