In the bowl of a stand mixer, blend egg whites and salt together in a bowl on medium speed until soft peaks form.
Meanwhile, bring corn syrup to a boil in a saucepan; add marshmallows and stir until melted.
Raise mixer speed to high. Very carefully, pour the hot syrup in a steady stream between the side of the mixing bowl and the moving beater. Beat until mixture is cool enough to touch; mix in vanilla extract. Makes enough to frost a 9x13-inch cake or an 8-inch layer cake.
