1. Position racks in the upper and lower thirds of the oven, and preheat the oven to 275°F. Line 2 baking sheets with parchment paper.
2. To make the meringue: If your sugar is not superfine, spin it in the food processor for 15 seconds. Mix the sugar and cornstarch together thoroughly. Combine the egg whites and vinegar or cream of tartar in a large, clean, dry bowl, and beat at medium-high speed (with a stand mixer) or at high speed (with a handheld mixer) until egg whites are creamy white and hold a soft shape when the beaters are lifted. Gradually add the sugar mixture a heaping teaspoon at a time, taking 2 1/2-3 minutes in all; you should have a very stiff, creamy meringue.
3. To make 8 individual pavlovas: Scoop 4 equal portions of the meringue onto each lined baking sheet, spaced well apart. Sculpt each portion into a small dome, about 4 1/2 inches in diameter. Bake 1 hour, until the pavlovas are golden pink and crusty on the surface and marshmallow-y within, rotating the sheets from top to bottom and front to back halfway through the baking time.
4. Set baking sheets on a rack to cool completely (pavlovas may sink a little). If you are not serving the pavlovas the same day, cover the meringues loosely and leave at room temperature; they keep for several days.
5. To assemble the dessert: In medium bowl, beat cream, with the vanilla, if using, until it is almost stiff. Top meringues with whipped cream and garnish with the fruit.