1/2 cup packed light brown sugar
1/2 cup packed dark brown sugar
1/4 cup milk
2 cups confectioners’ sugar, sifted
1 teaspoon pure vanilla extract
Place the butter and brown sugars in a heavy medium-size saucepan over medium heat. Cook, stirring, until the mixture comes to a boil, about 2 minutes. Add the milk, stir, and let the mixture return to a boil, then remove the pan from the heat. Add the vanilla and confectioners’ sugar 1/2 cup at a time. Beat with a whisk until the frosting is smooth. Add the last 1/2 cup of confectioners’ sugar, but not so much so that it thickens and hardens. Spoon a heaping tablespoon of frosting on top of each cupcake and spread it out with a short metal spatula or spoon, taking care to cover the top completely.