Fit your mixer with the whisk attachment. Put sugar and salt in the bowl. Add the vanilla and boiling water and whisk on medium-low speed until smooth and cool.
Add the shortening and the butter. Whisk until smooth.
Increase the speed and whip on medium-high speed until the frosting is very light and fluffy and has doubled in volume. This could take up to 15 minutes, so be patient. You can flavor this with other extracts, as well"”almond, butter, even peppermint.