In a deep, large kettle melt the butter over high heat until it just starts to boil.
Add the brown sugar.
Bring to a boil and lower heat to medium and continue to boil for 2 minutes.
Don't boil longer than two minutes and stir constantly while it is boiling.
Add the milk and splash of vanilla and return to a boil, stirring constantly.
As soon as it begins to boil, remove from heat and cool to lukewarm.
Gradually add confectioners' sugar.
Stir with a wooden spoon until thick and spreadable.