Beat the butter and/or shortening until fluffy., Beat in the salt, meringue powder, and vanilla., Add the confectioners' or glazing sugar and 2 tablespoons of the milk, and beat until smooth. Scrape the sides and bottom of the bowl., Adjust the consistency of the frosting as needed by adding more confectioners' sugar or milk., If you're not going to use the frosting right away, keep it at room temperature, covered, to prevent it from developing a dry crust.