Preheat the oven to 250 degrees. In the middle of a piece of parchment paper cut to fit a half-sheet pan, draw two 7-inch circles that are at least 2 inches apart. Place on a half-sheet pan, circle side down.
In a small bowl, mix together 1 tablespoon of sugar and the cornstarch; set aside.
With an electric mixer fitted with the whisk attachment, beat the egg whites, salt, and cream of tartar together on medium speed (4 to 6), until frothy.
Turn the mixer speed down to medium-low (3) and slowly add the remaining sugar to the egg white mixture, 1 tablespoon at a time.
Next, add the sugar-cornstarch mixture, then the vinegar and vanilla. Beat on medium speed (4 to 6) until the mixture is glossy, forms stiff peaks, and the sugar is dissolved – 10 to 15 minutes. (To test if it’s ready, gently rub a small amount between your fingers. If it feels smooth with no granules, it's good to go.)
Divide the meringue mixture evenly between the circles on your parchment paper – just plop it in the middle. Using a spatula, first smooth the outsides. Then slowly start spreading it out from the middle to around 1/2 to 1 inch from the circle. Next, take a spoon and spread the mixture so it’s slightly higher around the edges than in the middle. It should be around 7 inches in diameter now. Don't worry about making it perfect - the beauty of Pavlova lies in its rustic charm!
Bend down and look at the meringues from the side, to check and make sure they’re even.
Place in oven and bake for 10 minutes.
Reduce heat to 225 degrees and bake for an additional 70 minutes. When done, the meringues should be dry and crisp. It may crack around the edges, and that’s okay. Watch carefully starting at 60 minutes; if it starts to turn a light tan color, it’s probably done.
Turn off oven, leave the door closed, and leave meringues inside to cool for 1 1/2 hours. Prop the oven door open (I use a wooden spoon) and leave for another 30 minutes or until meringue is completely cool.
If not using immediately, carefully remove meringues from parchment paper, wrap airtight with plastic wrap, and store at room temperature until ready to use (up to 2 days). Can also be frozen for up to 1 month. Thaw overnight in the refrigerator – note that the consistency may change a bit.