2 tbsp. butter
2/3 c. dark brown sugar, packed
1/8 tsp. salt
1/3 c. evaporated milk
2 1/3 to 2 1/2 c. powdered sugar
1/2 tsp. vanilla

Heat butter in a 2-quart saucepan until melted. Stir in brown sugar, salt and evaporated milk. Heat to boiling, stirring constantly. Remove from heat, cool to lukewarm. Gradually stir in enough powdered sugar to reach desired spreading consistency. Stir in vanilla. Spread on cake.