Step 1Heat the oven to 350 degrees. Draw an 8-inch circle on a sheet of parchment paper and line a baking sheet with the parchment.
Step 2In a standing mixer fitted with a whisk or using a handheld mixer, beat the egg whites and salt on medium-high speed until peaks form, about 3 minutes.
Step 3With the machine on medium speed, whisk in the sugar a little at a time, then turn the machine back to high to fully incorporate.
Step 4Add the cornstarch and whisk to blend, then add the vinegar and, when incorporated, whisk in the vanilla.
Step 5Pile the meringue onto the parchment paper, using a spatula to spread the meringue around the circle and piling the sides slightly higher than the middle. Put into the oven and immediately reduce the temperature to 300 degrees.
Step 6Bake the meringue for 1 1/2 hours, then turn the oven off and prop open the door. Let the meringue cool down completely. It can be stored uncovered for several hours.