In the bowl of an electric mixer, combine powdered sugar and salt.
Using a whisk attachment, add boiling water and whip at low speed until smooth and cool, approximately 5 minutes.
Add shortening and margarine to the sugar mixture and whip at medium speed until smooth, approximately 3 minutes. Add vanilla. Increase speed to medium high and whip until light, fluffy and increased in volume, approximately 10 minutes. Frosting will fill most of a 5-quart mixing bowl.
Notes
TIP: Leftover “Buttercream” Frosting is great on cupcakes or cookies. Refrigerate in a sealed container for up to 2 weeks. Allow chilled frosting to return to room temperature before using. To fluff it up, whip for a couple minutes.