In the bowl of an electric mixer, fitted with the whisk attachment, beat butter (on medium speed) until completely smooth (30 seconds to a minute). (If you are adding cream cheese, add it now, then beat until smooth)
Add salt (if needed).
One cup at time, add 2 cups of powdered sugar, beating after each addition.
Add vanilla, and blend to combine.
Add about 1 tablespoon of cream, and continue beating.
Add the remaining 2 cups powdered sugar, one at a time.
Add an additional 1-2 tablespoons cream until desired consistency is reached. For the icing you'll be coloring and piping onto the cookies, you want the icing to be nice and thick, so you may not need to add the additional cream.
Continue to beat another minute or so. The frosting should now be smooth and fluffy.