In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium speed until smooth and creamy. With mixer on low speed, gradually add confectioners' sugar alternately with cream, beginning and ending with sugar, beating until combined after each addition and stopping to scrape sides of bowl. Beat in vanilla paste; scrape bottom and sides of bowl and paddle. Beat at medium-high speed until smooth, pale, and fluffy, 1 to 2 minutes. Use immediately.