Heat the flour and milk together until beginning to thicken then pour into a different container and cool to room temperature. (You can stick it in the freezer to speed the process.)
While the milk mixture is cooling, cream together the butter and sugar until extremely fluffy and the sugar is dissolved - this can take up to 10 minutes.
When the milk is room-temperature, add it to the creamed butter and beat for another 3 minutes or so.
Notes:
Make Ahead: This is a very stable frosting, so you could make it in advance and store it in the fridge.You can make this interchangeably with flour and cornstarch.