In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed until soft, about 1 minute. Add in 2 cups of the confectioners’ sugar. Then, add the salt, and vanilla.
Beat on low speed until the sugar is well incorporated. Add in the cream/milk. Add remaining 1-2 cups of sugar as needed to thicken the frosting. Beat on medium speed until the frosting is smooth and fluffy, about 4 minutes.