Add the granulated sugar, salt, cream of tartar, egg whites, and water to a stainless steel heavy-bottomed saucepan.
Do not add the vanilla extract at this stage.
Place the saucepan over medium-low heat.
Using an electric hand mixer, beat the mixture constantly while it heats.
Continue beating for 5 to 7 minutes, or until the frosting becomes thick, glossy, and fluffy.
Check the consistency by lifting the beaters from the frosting.
The frosting is ready when stiff peaks form and hold their shape.
Remove the saucepan from the heat.
Add the vanilla extract.
Stir until the vanilla is fully incorporated.
Use the frosting immediately on a completely cooled cake.
Spread the frosting evenly with a spatula or spoon, creating peaks or swirls as desired.
Serve the frosted cake the same day for the best texture.