In a saucepan, melt butter and stir in brown sugar. Bring to a boil and cook for 2 minutes over low heat, stirring constantly. Slowly stir in milk and bring to a boil. Remove from heat and let cool somewhat. Add powdered sugar a little at a time, beating until smooth and thick enough to spread. Add more sugar if necessary to obtain proper consistency. If icing becomes too stiff, add a little water.