Position oven rack in center of oven. Preheat oven to 300°F (150°C).

In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and salt at medium-low speed until soft peaks form, 5 to 6 minutes. With mixer on low speed, add castor sugar in a slow, steady stream, beating until combined. (This should take about 3 minutes.) Increase mixer speed to medium-high, and beat until thick and shiny, 10 to 12 minutes, stopping to scrape sides of bowl halfway through mixing. Rub meringue between 2 fingers to make sure it is smooth and no sugar granules can be felt.

In a small bowl, whisk together cornstarch, vinegar, and vanilla. Add to meringue, and beat at high speed until combined, about 30 seconds.

Using a permanent marker, draw a 7-inch circle on a 16½×12-inch sheet of parchment paper. Turn parchment over, and place on a rimmed baking sheet. Spoon meringue inside drawn circle. Using a spatula, shape meringue until it fills drawn circle, with straight sides and a flat top. (Try to do this quickly—the more you work with it, the harder it will be to shape later on.) Place tip of a small offset spatula at base of meringue, and pull up to create a long groove. Repeat around outside edge. Place meringue in oven.

Immediately reduce oven temperature to 225°F (107°C). Bake until dry and firm to the touch, 45 minutes to 1 hour. Turn oven off, and leave meringue in oven with door closed for 8 hours or up to overnight.

Measure ½ cup (120 grams) syrup from cherries. Bring to a boil over medium heat, and cook, stirring occasionally, until reduced by about half, 6 to 8 minutes. Transfer to a heatproof bowl, and let cool completely.

Spoon Chocolate Pastry Cream in center of meringue; top with Whipped Cream. Top with cooled cherry syrup and cherries. Garnish with chocolate curls, if desired. Serve immediately.