Cream butter and salt together in a stand mixer for 30 seconds. Softened means out of fridge for 30 minutes to an hour. It should not be mushy butter. Firm enough to push a finger indent in to butter. Add in cream cheese and mix just until combined. Scrape down sides of bowl with a rubber spatula.Add in sugar and mix for another 30 seconds. Add in egg and vanilla and mix for 30 seconds or less.