In a small saucepan, whisk flour into milk and heat, on medium, stirring constantly, until it thickens. You want it to be very thick. This takes about 4 minutes or so. Remove from heat and let it cool to room temperature. It must be completely cool before you use it in the next step. Stir in the vanilla.

While the mixture is cooling, cream the butter and sugar together until light and fluffy. Then add the cooled milk/flour/vanilla mixture to the butter and sugar and whip it on high speed until it all combines and looks like whipped cream.

This frosting needs refrigerated if temperature is over 70 degrees.