Homemade Marshmallow Frosting – your new favorite cake and cupcake icing! Made with just 5 common ingredients, this frosting is sweet, fluffy, and delicious toasted. Use it wherever you would classic buttercream or meringue topping for a sweet twist!
In a medium, heat-proof mixing bowl (that fits over a double-boiler), whisk together the egg whites, sugar, and cream of tartar for 1 minute until foamy. Scrape down the sides of the bowl with a spatula.
Fill the double-boiler with 1 inch of water and bring it to a simmer over medium-high heat. Reduce the heat to medium-low and place the bowl with the egg mixture over top, ensuring that the bottom of the bowl does not touch the water.
Cook the egg white mixture, stirring frequently, for 8 to 10 minutes until the sugar is dissolved and the temperature reaches 160°F. (Be careful, the mixture is very hot. You should not feel any sugar granules when rubbed between your fingers.)
Place the egg mixture in the bowl of a stand mixer fitted with the whisk attachment. Add the vanilla and salt. Beat on high speed (level 8) until thick and glossy and the sides of the bowl are just warm to the touch, 4 to 5 minutes.
Spread or pipe the frosting onto a cake or cupcakes. (This frosting should be used immediately as the meringue will start to set after it is whipped.) You can use a kitchen torch to toast this meringue-style icing, if desired.
Notes
Cook the egg whites to the proper temperature. The sugar and egg mixture is ready to whip when it reads 160°F with a candy thermometer, and the sugar completely dissolves. If you don't have a thermometer, you can rub a small amount between your fingers; it is ready once it feels very warm and smooth. If you feel any sugar granules, keep stirring it over the hot water.
Don't let the mixing bowl touch the simmering water. The bowl should rest over it, not in contact with it, allowing for even heat. If your stand mixing bowl fits over your boiler without touching the water, you can cook the mixture in it before whipping. But I actually prefer using a separate bowl to keep my mixing bowl from getting hot.
For a perfectly white frosting, use clear vanilla extract. Natural vanilla extract is bourbon-colored, adding a light golden tone to the homemade frosting.