Step 1: To a medium sized heavy-duty saucepan over medium heat, add 8 tablespoons butter, 1/2 cup light brown sugar and 1/2 cup dark brown sugar. Stir and cook until the mixture comes to a boil, about 2 minutes.
Step 2: Add 1/4 cup milk, stir and bring the mixture back to a boil, then remove the pan from the heat.
Step 3: Add 2 cups confectioners’ sugar and 1 teaspoon pure vanilla extract. Beat with a wooden spoon or wire whisk until the frosting is smooth (it will be thin).
Step 4: Pour frosting, while still warm, over the cooled cake. Cut into squares and serve.