In a double boiler, combine the water, corn syrup, sugar, and cream of tartar. Heat over medium heat, stirring constantly, until the sugar is dissolved, about 3 minutes.
In a medium bowl, whisk together the egg whites and vanilla extract until foamy.
Transfer the egg white mixture to the double boiler and whisk until the mixture is thick and creamy, about 4 minutes.
Remove from the heat and whisk until the frosting is light and fluffy, about 1 minute.
Spread the frosting over the cake or cupcakes. Enjoy!
Interesting Facts
This seven-minute frosting is a classic American recipe.
The cream of tartar adds stability to the egg whites, which helps the frosting hold its shape better.
This frosting is best used immediately, as it will start to harden and lose its light and fluffy texture after an hour.