1 Preheat the oven and prepare the baking sheets: Preheat the oven to 400ºF and line two baking sheets with silicone baking mats or parchment paper.
2 Make the filling: In a very clean, large mixing bowl, use a whisk, hand mixer, or stand mixer with whisk attachment to whisk together the egg whites and cream of tartar, just until the cream of tartar is fully incorporated and the egg whites have a lot of bubbles.
Add the sugar to the egg whites, one tablespoon at a time, whisking it in completely after each addition, for a minute or so if you’re doing it by hand, or about 30 seconds if you’re using an electric mixer.
Add the vanilla, then continue to whisk at high speed, until the egg whites are glossy and silky in texture. Stop the mixer and rub a little of the meringue between your fingers. It should feel smooth and you should not be able to feel any gritty sugar between your…