Combine milk and flour in pot and heat to boiling over medium heat. Boil for 2 minutes, stirring constantly. Cool to room temperature.
Combine butter, sugar and vanilla. Beat until fluffy and very light.
Mix cooled Mixture with butter batch. Beat on high until it looks like whipped cream and is spreadable. If you can still see sugar when the frosting is rubbed between your fingers then it is not done. Be careful not to overbeat.
For chocolate buttercream add 3 Tbsp. Baking Cocoa with the milk and flour.
For butterscotch buttercream substitute 1 Cup brown sugar for the sugar.