Whisk flour into milk in a small saucepan over low heat. Stir constantly until it thickens. It should be very thick, like brownie mix. Remove from the heat and let cool completely to room temperature. (I sped this process along by placing the pan in a bowl filled with ice.) Stir in vanilla.
While the mixture is cooling, cream the butter and sugar together until light and fluffy. Make sure you beat it until there is no graininess left from the sugar. Then add the cooled milk/flour mixture and beat it. A lot! If it looks separated, you haven’t beat it enough. It’ll resemble thick whipped cream.
Taste it. Die and go to heaven.
Hurry up and frost the brownie bites or cake or whatever you’re frosting before you eat it all with a spoon.