Beat the icing sugar and butter together in a freestanding electric mixer with a paddle attachment (or use a handheld electric whisk) on medium-slow speed until the mixture comes together and is well mixed. Add the cream cheese in one go and beat until it is completely incorporated. Turn the mixer up to medium-high speed. Continue beating until the frosting is light and fluffy, at least 5 minutes. Do not overbeat, as it can quickly become runny.