Melt butter in a medium saucepan over medium heat.  Stir in brown sugar.  Bring to a boil, stir constantly.  Heat to boiling, reduce heat to low and boil for two minutes, continue stirring.  Stir in milk and salt.  Bring to a boil, remove from heat.  Let cool for 30 minutes.  With an electric hand mixer mix in 2 cups of the powdered sugar.  It will look thick and crumbly.  Continue beating.  Add 1 tablespoon of milk and the additional cup of powdered sugar.  Beat until smooth and fluffy.  Add milk one tablespoon at a time to reach desired consistency.  Pipe or spread on cupcakes.