Separate the eggs whites and yolks making sure there is no yolk in the egg whites. Reserve the yolks for another use (like ice cream!) Lightly cover the egg whites and leave at room temperature for a hour or two.
When ready to make the Pavlova, preheat oven to 340F. Line a baking sheet with parchment paper. Using a 9" cake pan as a guide, draw a circle on the parchment paper. Turn the paper over and set aside.
Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. (Alternatively use a hand mixer) Beat on medium high until the egg whites are very foamy and hold soft peaks.
Add the sugar, one tablespoon at a time, blending for a few seconds after each addition. After adding all the sugar, beat for 3-4 minutes or until the meringue is no longer gritty. To test, rub a small dollop between your fingers.
Sift the cornstarch over the meringue, and add the salt and vinegar. Fold the mixture with a rubber spatula until all ingredients are incorporated.
Take about 1 teaspoon of the meringue and use it to secure the parchment paper to the baking sheet. Place a small dot under the 4 corners and press against the pan to adhere. This makes it much easier to spread the meringue into the circle.
Dollop half the meringue onto the parchment and using a large metal spoon, carefully spread to fit the circle. Add the remaining meringue on top and spread almost to the edges. The sides should be slightly sloping. Spread to flatten the top. The whole Pavlova should be about the size of a 9-inch cake pan and about 2-inches high with sloping sides, wider on bottom, slightly smaller on top (see photos.)
Carefully place the Pavlova in the oven and immediately reduce the temperature to 240F. Bake for 90 minutes without opening the oven. After 90 minutes, turn off the oven (leaving it closed) and allow the Pavlova to cool for several hours.