Melt butter in a 2-quart saucepan; stir in brown sugar and salt. Bring to a boil and boil hard for 2 minutes, stirring constantly.
Remove from heat and add milk, stirring vigorously.
Put back over heat and bring to a full boil again. Set aside to cool until lukewarm.
Stir in powdered sugar and beat until smooth and cool enough to spread. Add more powdered sugar as necessary to achieve the desired consistency.
Work quickly to frost your cake or cupcakes, as it will become stiffer as it cools. If frosting stiffens too quickly while spreading, beat in a few drops of milk.
Frosts tops and sides of two 8- or 9-inch layers.