Brown Sugar Frosting

5 tablespoons (2 1/2 ounces) unsalted butter
1/2 cup (3 1/4 ounces) firmly packed brown sugar (light or dark) 
3 tablespoons (1 1/2 ounces) milk
1 1/2 cups (6 ounces) confectioners' sugar
1/2 teaspoon vanilla 

Melt the butter in a small pan over medium heat. Stir in the brown
sugar and cook, stirring, until the sugar melts. Add the milk, bring
to a boil, and pour into a mixing bowl to cool for 10 minutes.

After 10 minutes, stir in the confectioners' sugar and vanilla. Beat
well; if the mixture appears too thin, add more confectioners'
sugar. Spread on the cake while frosting is still warm.

[From http://www.kingarthurflour.com/recipes/old-fashioned-apple-cake-with-brown-sugar-frosting-recipe]