I got this from the Top Secret Recipes site, and I think it is even better than the original-YUMMY!

2 tsp very hot water
1/4 tsp salt(rounded)
7 oz jar marshmallow creme
1/2 cup shortening
1/3 cup powdered sugar
1/2 tsp vanilla

Combine the salt with the hot water in a small bowl and stir until salt is dissolved. Let this mixture cool. Combine the marshmallow creme, shortening, powdered sugar, and vanilla in a medium bowl and mix well with an electric mixer on high speed until fluffy. Add the salt solution to the filling mixture and combine. Makes 1 1/2 cups, enough to generously fill a layer cake.