Set oven to 300F. Use a plate, bowl or cake pan to draw a circle between 7 and 8 inches (18-20cm) on a sheet of parchment paper. Place the paper drawn side down on a baking sheet and set aside.
Separate the four egg whites into a small bowl. You can reserve the yolks for a batch of French buttercream, custard, or pastry cream.
Add the egg whites, cream of tartar, vanilla, and salt to the bowl of a clean stand mixer fitted with a whisk attachment and mix on medium-low. You can also use an electric hand mixer but if will require some patience.
Once the egg whites are frothy you can begin sprinkling in the sugar a tablespoon at a time. The key is not to dump the sugar in but to add it very slowly. This gives you a very thick, glossy meringue with a marshmallowy texture.
Sprinkle the cornstarch onto the meringue and fold in using a spatula
Mix on high until stiff peaks form. Once you're getting close it's a good idea to scrape the bowl down so you don't have any less mixed bits at the bottom of your bowl.
Transfer the meringue to your prepared baking sheet and use a spatula to shape it into a disk with a slight depression on top. You can use a small spatula to create vertical swoops on the side or leave things nice and rustic.
Place your meringue into the oven preheated to 300F then immediately reduce the temperature to 250F and bake for about for 1 hr-1hr 15 minutes. The meringue will turn ever so slightly cream in color and feel dry and hard, the inside will remain soft however.
When your meringue is done turn the oven off and leave the door closed for an hour. You can then crack the door open for an hour. You're trying to let the meringue come to room temperature slowly, in fact you can leave it in the oven overnight if you're concerned about humidity messing with things.