Preheat oven to 300°. Line a sheet pan with parchment paper, trace a 9-inch circle in the center (we used a cake pan as our guide), and turn the paper over.

In the clean bowl of a standing mixer with a whisk attachment, beat egg whites until frothy. Add salt and continue beating. As soft peaks form, slowly add sugar in a thin stream, beating as you go. Once sugar is incorporated, beat on high speed until firm, shiny peaks begin to form, about 2 more minutes. Stop beating; then gently fold in vanilla, vinegar, and cornstarch with a spatula.

Spoon the meringue into the center of the traced circle and use the back of the spoon to spread it out to the edges, creating a shallow well in the middle. Put into the oven, reduce heat to 250°, and bake 1-½ hours. Turn off the oven, leaving meringue inside until it turns crisp and pale, but still a bit soft inside, about 1 hour. Remove from oven and let cool completely.