Combine the milk and flour in a heavy saucepan with a wire whisk. Cook over medium heat, whisking constantly until the flour is completely dissolved and the mixture is very thick (to the point you can barely stir it - switch to a wooden spoon if needed). Remove the pan from the stove and let the mixture cool completely. Meanwhile, in a large mixing bowl, combine the butter, sugar, and cocoa (if using) and beat with a mixer on medium speed until creamy and combined. Add the thickened milk mixture and vanilla extract and beat on medium speed for 4-5 minutes or until thick and creamy. Frost a cooled cake or cupcakes as desired. The frosting can also be tinted with food coloring.