Prepare a Double Boiler: Use a double boiler, or fill a pot with an inch or two of water and place a heatproof mixing bowl on top to create a double boiler.
Heat the Mixture: On low heat, add the sugar, salt, and egg whites to the bowl. Whisk continuously until the sugar dissolves and the mixture becomes hot to the touch, about 5 minutes.
Beat the Icing: Pour the mixture into a large mixing bowl or the bowl of a stand mixer. Add the vanilla extract and beat on high speed until stiff peaks form, about 7 minutes.
Ice the Cupcakes: Spoon the marshmallow icing onto cooled cupcakes.
Optional: For a toasted marshmallow finish, place the iced cupcakes under your stove’s broil setting for 30 seconds or use a kitchen torch to lightly toast the marshmallow.