Preheat the oven to 225°F and line two baking sheets with parchment paper.
Make sure your bowl and whisk are completely clean and the egg whites don’t have any egg yolk in them. Any small amount of fat will affect the results of the meringue.
If you haven't already, separate the egg whites from the egg yolks. You can do this by passing the yolk back and forth between two halves. It's easier to separate egg white when the eggs are cold, but you want the egg whites room temperature to make the meringue. You can separate the egg white then allow them to come to room temp for 30 minutes before whisking.
In a stand mixer fitted with the whisk attachment or a hand mixer, whisk the egg whites on medium speed until the mixture starts to foam and forms soft peaks. Add the cream of tartar, salt and vanilla and whisk until combined.
Turn the mixer up to medium-high and continue to whisk as you slowly add the sugar 1-2 tablespoons at a time, beating well after each addition until all the sugar is fully incorporated.
Once all of the sugar has been added, beat until sugar has just dissolved and the mixture is glossy and stiff peaks form, about 4 minutes. You can't over-beat meringue so be sure it's stiff and glossy and there isn't any grain when you rub the meringue between your fingers.