Stir the sugar, water, cream of tartar, and salt together in a small saucepan. Heat over medium high heat until the mixture boils. Wash down any sugar crystals that are stuck on the side of the pan. Continue cooking until sugar mixture reaches 242°.
While the sugar syrup is boiling, place the egg whites in a large mixing bowl. Just before the sugar syrup reaches the proper temperature, start beating the egg whites at high speed until soft peaks form.
With the mixer still on high speed, pour the hot syrup very slowly in a thin stream into the egg whites. Add the vanilla and continue beating until the frosting is thick and white and holds its shape.
Immediately spread the frosting on a cooled cake. This frosting will be pillowy soft and spread easily. Use your spatula or the back of a spoon to make beautiful swirls in the frosting.
Decorate your cake as desired. This frosting is best served the same day it is made and will last up 2-3 days at room temperature. Enjoy!
Notes
Recipe Tips:
Do not make the frosting too far ahead. This frosting is best served the same day it is made. It is much more delicate than other types of frosting (like buttercream) and will slowly deflate over time. It will last 2-3 days before it starts to get pretty flat.
Start with clean equipment. Make sure there is no grease on your beaters or mixing bowl. Any trace of fat may prevent the eggs from beating properly.
Separate the eggs carefully. Egg yolks contain fat, which can prevent the egg whites from beating up properly. Inspect each egg carefully for any traces of yolk.
Cook the sugar syrup to the proper temperature. Make sure to check the temperature with a candy thermometer. This will ensure that the frosting is the proper consistency.