In the bowl of a stand mixer fitted with the paddle attachment, beat butter at medium speed until creamy, 1 to 2 minutes. With mixer on low speed, gradually add confectioners’ sugar and salt, beating until smooth and stopping to scrape sides of bowl. Add buttermilk and vanilla bean paste, beating until smooth and stopping to scrape sides of bowl. Increase mixer speed to high, and beat until light and fluffy, about 2 minutes. Use immediately.