Preheat oven to 200 degrees Fahrenheit. Line a baking sheet with parchment paper.
In a bowl with an electric mixer, combine egg whites and salt. Begin beating at low speed, slowly increasing to high. Continue until peaks begin to form; gradually beat in sugar and cream of tartar. Meringue should be stiff and shiny. Add cornstarch and white wine vinegar and mix gently to incorporate.
Spoon onto parchment, creating an 8-to-9-inch circle shape or disc. Build the sides higher than the middle and smooth out sides with a knife.
Place in oven, and immediately reduce heat to 180 degrees. Bake 1.5 hours. Turn off heat and allow the meringue to cool completely in oven overnight. It will be crisp on the outside and soft on the inside.