In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium speed until creamy, 1 to 2 minutes, stopping to scrape sides of bowl. With mixer on low speed, gradually add confectioners’ sugar alternately with buttermilk, beginning and ending with confectioners’ sugar, beating just until combined after each addition. Beat in vanilla. Increase mixer speed to medium; beat until fluffy, 2 to 3 minutes, stopping to scrape sides of bowl. Use immediately.