Place oven rack in the center of the oven. Preheat oven to 180 degrees. Grab a sheet of parchment paper and track a 9-inch circle in the center. Turn over the paper and place onto a baking sheet. Set aside.
Add egg whites to the bowl of a stand mixer with salts. Using the whisk attachment, beat egg whites on medium high until the start to get foamy. At that time, with the mixer on medium, add sugar 2 tablespoons at a time, whisking for about 30 seconds before adding the next addition. This ensures the sugar gets completely dissolved into the egg whites. Once all the sugar has been added, take the speed up to medium high and whip into stiff peaks. The egg whites should be fluffy and glossy. Stop and scrape down the sides of the bowl to incorporate all the sugar and then beat for 15 more seconds.
With the mixer turned off, sift the cornstarch over the stiff egg whites. Add the white wine vinegar and vanilla extract and fold all three into the egg whites gently. Spoon the beaten egg whites onto the traced 9-inch circle of the parchment paper from Step 1. Spread egg whites into an even disk about 2 inches high. Place in preheated oven to bake for 90 minutes. Do not open oven door for any reason. You don't want to disturb the pavlova while it bakes.
After it's baked for 90 minutes, turn off the oven but leave the pavlova in the oven to cool for 1 hour. In the meantime, prepare the berries and whipped cream.