Penuche Frosting

In a sauce pan melt 1/2 cup butter (no substitutes). Stir in 1 cup packed brown sugar. Cook and stir until bubbly. Remove from heat. Add 1/4 cup milk; beat vigorously with a wooden spoon till smooth. Add 3 1/2 cups powdered sugar, beat by hand till it reaches spreading consistency. Immediately frost the cake as frosting does harden!