Measure out sugar, cold water, egg whites, salt, vanilla extract, and cream of tartar. Having everything ready will make the process smoother.

In a medium heatproof bowl, combine sugar, water, egg whites, salt, and cream of tartar. Stir gently until the sugar starts to dissolve, then use an electric mixer on low speed to beat for about 30 seconds.

Bring a medium saucepan with a few inches of water to a simmer. Place the bowl with the egg white mixture over the saucepan, ensuring the bottom of the bowl does not touch the water.

Using an electric mixer on high speed, beat the mixture over the simmering water for 7 minutes, or until it becomes thick, glossy, and forms stiff peaks.

Remove the bowl from the heat. Stir in vanilla extract and beat for an additional 2 minutes to ensure the frosting is smooth and silky.

Spread the frosting immediately over cakes or cupcakes, creating decorative swirls or peaks. The frosting will set quickly, forming a delicate crust.