Melt ½ c. butter in saucepan over low heat. Stir in 1 c. dark brown sugar, firmly packed, and ¼ tsp. salt. Bring to a boil over medium heat; boil hard 2 minutes, stirring constantly. Remove from heat. Stir in ¼ c. milk. Return pan to heat and bring to a full boil. Remove from heat, cool to lukewarm. Stir in 2 c. confectioners sugar and beat until smooth. If frosting is too thick, beat in a little milk.