Melt 1/2 cup butter in a saucepan over low heat. Stir in 1 cup dark brown sugar, firmly packed, and 1/4 tsp. salt. Bring to a boil over medium heat; boil hard for 2 minutes stirring constantly. Remove from heat. Stir in 1/4 cup milk. Return pan to heat and bring to a full boil. Remove from heat, cool to lukewarm. Stir in 2 cups confectioner's sugar and beat until smooth. If frosting is too thick, beat in a little milk.
I didn't have the topping ingredients for an apple cake when I baked it, so I just used this frosting later after I bought some brown sugar.
Source: Farm Journal Cookbook, 1959 edition