Preheat oven to 250°F.Cover cookie sheet with parchment paper. Using a large round pan, draw a circle on the parchment paper. Add egg whites to bowl of electric mixer, and beat until soft peaks form; very gradually add sugar. Continue to beat until egg whites are very stiff, continue to beat adding salt, vinegar and vanilla slowly. Add cornstarch and keep beating until mixture is really glossy and will stand when cut with a knife. Remove egg white mixture from bowl and place in the middle of circle on parchment paper and spread it into a circle. Make a small indentation in the center. Place the cookie tray in the middle of the oven and cook for 1 hour; turn oven off.