Recipe(tried): Butterscotch-Chocolate Frosting for Sourdough Chocolate Cake

I have the same recipe as Marta for Sourdough Chocolate Cake, but mine, clipped from the newspaper, includes the recipe for this yummy frosting.

Butterscotch-Chocolate Frosting for Sourdough Chocolate Cake

3 squares (1 oz each) unsweetened chocolate
(or 9 Tablespoons cocoa plus 3 Tablespoons butter)
1/4 cup butter or margarine
1/4 cup half and half(or evaporated milk)
1/4 cup packed brown sugar
1/4 teaspoon salt
1 teaspoon vanilla extract
About 3 cups powdered sugar

In a large saucepan, combine chocolate, butter, half and half, brown sugar and salt. Stir over medium heat until mixture almost comes to a boil and chocolate melts. Stir in vanilla. Gradually beat in enough powdered sugar (about 3 cups) to make a smooth, spreading consistency.