In the work bowl of a stand mixer add the water and salt. Stir to dissolve the salt in the hot water and allow to cool.
Add the marshmallow cream, shortening, powdered sugar, and vanilla to the salt water mixture and whip on medium speed until the mixture becomes fluffy fluffy.
Recipe by Evil Shenanigans at https://evilshenanigans.com/2009/02/cream-filling-for-cupcakes/