Preheat oven to 250° F and set oven rack to the middle position.
Beat the egg whites and when soft peaks form add sugar very gradually.
Continue to beat and when very stiff slowly add salt, vinegar and vanilla.
Add the potato starch and keep beating until the mixture is really glossy and will stand up with a knife.
Cover a cookie sheet with parchment paper.
Tip the mixture into the center of the cookie sheet and spread it into a circle about 8"-9" in diameter. Leave a slight depression in the center area.
Put cookie sheet in oven, bake for 1 hour, then turn off oven and leave the pavlova in it until the oven is cooled.
Put on serving plate and top with whipped cream and then cover with whatever fruit is in season.