1 stick butter
1 cup dark brown sugar
⅓ cup milk
1 tsp vanilla
2 to 3 cups powdered sugar
Put butter, milk, and brown sugar in saucepan. Bring to a boil while stirring. Reduce to simmer and cook 3-5 minutes, the mixture will be foaming. Remove from heat and add vanilla. Pour into mixer bowl and beat while adding powered sugar until desired stiffness.
While hot the icing will be flowable, much like cake batter. As it cools it will stiffen up. For best results ice the cake while the icing is still warm.
Makes enough for a two layer cake. If splitting layers increase by 50%.