In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium speed until smooth and creamy, 1 to 2 minutes, stopping to scrape sides of bowl. With mixer on low speed, gradually add confectioners’ sugar, milk powder, and espresso powder, beating just until combined. Beat in reduced Guinness and vanilla; scrape sides of bowl. Increase mixer speed to medium, and beat until fluffy, about 2 minutes, stopping to scrape sides of bowl. Use immediately.