Found this when I did a search.
Quick Caramel Frosting
8 tablespoons (one stick butter)
1/2 cup packed light brown sugar
1/2 cup packed dark brown sugar
1/4 whole milk
2 cups confectioners' sugar, sifted
1 teaspoon pure vanilla extract
1. Place the butter and the brown sugars in a medium-size heavy saucepan over medium heat. Stir and cook until the mixture comes to a boil, about two minutes. Add the milk, stir and bring the mixture back to a boil, then remove the pan from the heat. Add the confectioners' sugar and vanilla. Beat with a wooden spoon until the frosting is smooth.
2. Use immediately (while still warm) to frost the cake of your choice or the frosting will harden. If it does harden while you are frosting the cake, simply place the pan back over low heat and stir until the frosting softens up.