Bring to a boil, lower heat to medium low, and continue to boil for 2 minutes, stirring constantly.
Add the milk and return to a boil, stirring constantly.
Remove from heat and cool to lukewarm.
Gradually add sifted confectioners' sugar. (I would highly recommend sifting this confectioners' sugar. It's such a pain to sift, but it ensures that the frosting will be extra smooth.)
Beat until thick enough to spread. If too thick, add a little hot water.