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 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.
Notes
Tips
Whip over your double broiler until it's 160°F or until you don’t feel granules.
If you're using egg whites from a carton, note that one egg white from a large egg generally equals about 2 tablespoons. *Note that we have not tested packaged egg whites in this recipe.
Allow cake, cupcakes, cookies or brownies to cool completely before icing.
Use a storage bag with the corner clipped off or icing bag to pipe, though this frosting will not be set up enough to pipe like a traditional buttercream.
How to Store
At Room Temperature – you can store this marshmallow frosting out on the counter for up to two hours while serving.
Refrigerate – Because this icing has egg whites, you’ll want to refrigerate in an airtight container. It can be stored in the fridge for up to 48 hours. Note that the texture will change though, and a bit of a crust can form on the exterior of the frosting, while the interior remains soft.