Now that you have your butter and powdered sugar mixed together, it's time to add in some milk - but not too much, because, as we all know, too much milk can make your icing go from smooth and creamy to soupy and sad. You want it to be just right, like Goldilocks, not too hot, not too cold, but just perfect.
As you're mixing all these ingredients together, you might be thinking, "Wait, what about the heavy cream?" And to that, I say, who needs heavy cream when you have plain old milk?! It's like the unsung hero of the icing world - it gets the job done without all the fuss and calories of heavy cream.