In the bowl of a stand mixer fitted with the paddle attachment (or large bowl if using a hand mixer), combine the cream cheese, butter, vanilla and salt. Mix on low speed until combined, then increase the speed to medium-high and beat until aerated and light, about 2 minutes. Stop the mixer and add 2 cups of the confectioners' sugar; mix on low to combine. Mix in the remaining sugar in two additions, keeping the speed on low. Once all of the sugar is mixed in, increase the speed to medium-high and beat until fluffy, about 1 minute.