Preheat the oven to 225°. Line 2 large baking sheets with parchment paper. In a standing mixer fitted with a whisk or using a handheld electric mixer, beat the egg whites to soft peaks. With the mixer on, gradually add the granulated sugar and beat at medium speed until stiff, glossy peaks form, about 5 minutes. Reduce the speed to low and beat in the cornstarch and vinegar. Increase the speed to high and beat for 20 seconds. Spread the meringue on the prepared baking sheets in 9-inch rounds about 1/2 inch thick. Bake for about 3 hours, until the meringue is dry throughout and crisp. Let the meringue cool completely.