(thanks tasty kitchen)
In small saucepan add 5T of flour to 1 cup of milk (preferably whole). Stirring constantly until it makes a thick paste. Stir in 2 tsp vanilla and set aside.
Meanwhile, whip together on high 1 cup of softened butter (Not melted) and 1 cup of WHITE sugar (NOT POWDERED).
When paste is completely cooled, add it to the butter/sugar mixture and whip together for about 2-3 minutes until no longer grainy.
*at this point you can add in 1 cup of whipped cream cheese if you want. Room temp is best
Do not make ahead and refrigerate - it turns hard and tastes weird
use only on completely cooled cakes or it will melt.