1. Preheat oven to 250°F (130°C) and place rack in center of oven. Line a baking sheet with parchment paper and draw a seven inch (18 centimeter) circle on the paper.
2. Pour the vanilla and vinegar into a small cup. Stir the cornstarch into the sugar in a small bowl.
3. In a large bowl of a heavy-duty mixer, fitted with whisk attachment, whip egg whites and salt, starting on low, increasing incrementally to medium speed until soft peaks/trails start to become visible, and the egg white bubbles are very small and uniform, approximately two to three minutes.
4. Increase speed to medium-high, slowly and gradually sprinkling in the sugar-cornstarch mixture. A few minutes after these dry ingredients are added, slowly pour in the vanilla and vinegar (if you didn’t use cream of tartar.) Increase speed a bit and whip until meringue is glossy, and stiff peaks form when the whisk is lifted, four to five minutes.
5. Gently spread the meringue inside the circle drawn on the parchment paper, smoothing the edges, making sure the edges of the meringue are slightly higher than the center. (You want a slight well in the center of the meringue to place the whipped cream and fruit.)
6. Bake for one hour 15 minutes or until the outside is dry and takes on a very pale cream color. Check on meringues at least once during the baking time. If they appear to be taking on color or cracking, reduce temperature 25 degrees, and turn pan around. Turn the oven off, leave the door slightly ajar, and let the meringue cool completely in the oven. (The outside of the meringue will feel firm to the touch, if gently pressed, but as it cools you will get a little cracking and you will see that the inside is soft and marshmallowy.)
You can make the meringue cake several days in advance. Just store in a cool dry place, in an airtight container. However, once the whipped cream and fruit are placed on the meringue, the dessert should be eaten immediately as the meringue will start to soften and break down from the moisture of the cream and fruit.