Add 2 inches of water into a medium sauce pan and bring the water to a simmer on medium heat. Place the egg whites, cup of sugar, cream of tartar, and salt in a heat proof bowl and whisk to combine. Transfer the bowl onto the sauce pan with hot water (make sure the water is NOT touching the bowl) and whisk continuously until the sugar has completely dissolved-check with you fingers. Once the sugar has melted transfer the egg white mixture to a stand mixer and whisk on medium high speed for about 5 minutes. Until it turns fluffy, white and glossy! Add the vanilla and mix quickly until incorporated.