I know it sounds gross at first, but stick with me here..
What you need..
* I always make double because it is AWESOME!
In small saucepan cook flour and milk until it forms a ball (kind of like making gravy), stirring constantly. Cool to room temperature. I like to fill a large mixing bowl with ice and put the saucepan in it while I do the rest. It cools much faster, just stir a little.
With an electric mixer, beat softened butter and sugar until fluffy (A lot!).
Beat both mixtures together on high speed until fluffy and smooth. Add vanilla and beat until well combined. I've found you can't over beat it. Refrigerate for about 1/2 hour, until it is spreading consistency.
I'm messy :)