Make a double boiler by choosing a heat-safe metal or glass mixing bowl that can sit on top of a saucepan. Place 2 inches of water in the bottom of the saucepan. The bottom of the bowl should not touch the water. Bring the water in the saucepan to a boil. Once boiling, reduce the heat to medium or medium-low and leave it at a simmer.
Add the sugar, water, egg whites, cream of tartar and salt to the mixing bowl. Using an electric mixer, beat the mixture for 30 seconds on low speed until combined.
Place the mixing bowl over the simmering water and cook while mixing with the electric mixer on high speed for 7 minutes or until the frosting forms stiff peaks.
Remove the bowl from the heat and mix in the vanilla. Mix for another 2-3 minutes until the consistency is spreadable.
Working somewhat quickly, frost the top and sides of 1 angel food cake, or two 8 or 9-inch layer cakes. The frosting will set as it cools. Enjoy!
Make sure you whip the frosting enough so it forms stiff peaks. After the 7 minute cook time you'll want to whip it for an addition 2-3 minutes.
You'll want to work quickly to frost the cake once it's ready. Make sure the cakes are cooled and ready to go before you start making the frosting.
You can pair this frosting with any cake or cupcakes, including devil's food cake, coconut cake, or any other dessert that calls for a Swiss meringue recipe.