Boil the can of sweetened condensed milk ahead of time to give it time to cool. I've found cooking it with the crock-pot method (4 hours on high) is best.
Cool the can in ice water or keep in the refrigerator until ready to use. The sweetened condensed milk will have a caramel color after it is cooked.
You can use either a stand mixer with a paddle attachment or a hand mixer: Beat the cream cheese on high until it is light and fluffy.
Add the sweetened condensed milk and the cool whip and beat on medium-high until it is light and fluffy (about 6-7 minutes) - Don't over-mix.
Spread and enjoy as your new favorite cake frosting! Enjoy!