Combine egg whites, granulated sugar, and cream of tartar in a heat-safe bowl. (You can use the metal bowl of your stand mixer). Whisk to combine. Place the bowl over a saucepan of simmering water on the stovetop.
Continue to whisk, gently heating the egg white mixture. Cook for about 5 to 10 minutes until the sugar is dissolved. Test this out by rubbing a small amount of the egg white mixture between two fingers. If it is smooth and not grainy, it is ready.
Transfer the bowl or mixture to a stand mixer fitted with a whisk attachment. You can also use an electric hand mixer.
Whip the egg whites on low speed for 2 minutes until frothy. Increase the speed to medium and whip until the meringue thickens.
Add vanilla extract. Continue to whip until the meringue forms medium/stiff peaks. The time this will take may vary, but typically it is about 5 minutes or so. The meringue will appear glossy and maintain shape. Be careful not to over whip the meringue or it will break.
Spread or pipe the meringue frosting immediately for best results.