Heat oven to 250 degrees F; line a large baking sheet with parchment paper.
Blend sugar on high in a food processor for 15 seconds.
Measure sugar again. (You need 1 cup; process more, if needed.)
In a small bowl, combine cornstarch and 2 tablespoons sugar.
Place egg whites in the bowl of a stand mixer, and beat on medium-high speed until foamy peaks form, about 1 to 2 minutes.
Gradually add plain, processed sugar, 1 tablespoon at a time, to the egg whites, beating until incorporated, about 1-2 minutes.
Add cornstarch mixture, 1 tablespoon at a time, beating until whites are stiff and glossy, about 1-2 minutes.
Add vinegar and vanilla; beat for 1 minute.
Using a spatula, spoon meringue into 8 mounds onto prepared baking sheet.
Using a butter knife spread each into 3-1/2" disks.
After that, using the back of a spoon, make an indent in the center of each.
Bake for 40 minutes.
Turn off the oven, and prop the door open, placing a wooden spoon in the opening.
Let pavlovas cool in the oven for an hour.
Combine blueberries and strawberries in a small bowl; sprinkle with sugar.
Stir well; allow to sit for at least an hour
Transfer the pavlovas from the baking tray to a cooling rack, allow to cool fully.
Top with berries, if desired.