Use a double boiler or fill a pot with an inch or two of water and add a heat-proof mixing bowl to create a double boiler.
On low heat, add sugar, salt, and egg whites, whisking continuously until the sugar dissolves and the mixture is hot to the touch (about 5 minutes).
Pour mixture into a large mixing bowl or stand mixer and add vanilla. Beat on high until stiff peaks form, about 7 minutes.
Spoon marshmallow icing onto cooled cupcakes.
Optional: Toast marshmallow under your stove’s broil setting for 30 seconds, or using a torch.