Position a rack in the center of the oven. Preheat the oven to 250°F/120°C. Line a rimmed baking sheet with parchment paper, and (this isn't necessary, but I find it helpful) use a pencil to trace around an 8-in/20-cm cake pan on the parchment paper. Flip the paper over, and set the baking sheet aside.
In a large, clean bowl using a handheld mixer or in the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites on the lowest setting. After 15 seconds, gradually increase the speed to the highest setting. Once the mixture begins to foam, add the sugar and salt in a slow, steady stream.
The egg whites will begin to look glossy. Keep beating on high until firm (stiff) peaks form. The mixture should not feel granular when you rub it between your thumb and index finger, and the peak clinging on the wire whisk should stand with the utmost confidence. Fold the vinegar into the meringue with a metal spoon to avoid deflating, then spoon the meringue onto the prepared baking sheet, spreading it out into the drawn circle (if you're free-handing it, try to keep it around 8 in/20 cm in diameter and 2 in/5 cm in height). Bake for 1 hour and 15 minutes, until the outside of the meringue has hardened and the center is still soft and chewy.
Allow the meringue to cool on its pan. Peel off the parchment and transfer the meringue to a plate or cake stand.
Prepare the Billowy Whipped Cream (directions below). Spoon the whipped cream onto the pavlova, causally spiral the jam onto the whipped cream (if using), then top with the fruit of your choice and maybe a smattering of pistachio nuts before serving. Serve right away.
The pavlova shell will keep, unfilled, in an airtight container at room temperature for up to 2 days. A filled pav is best eaten on the same day that you make it. I know this goes without saying, but it's better to be safe than sorry: don't get any yolk in your egg whites; it'll prevent the meringue from firming up. The same goes for using greasy mixing materials.
For the whipped cream:
Put a large bowl in the freezer to chill for several minutes before beginning. In the chilled bowl, using a balloon whisk or a handheld mixer (or in the bowl of a stand mixer fitted with the whisk attachment), whip together 1/2 cup/120 ml of the cream and the confectioners' sugar until firm, stiff peaks form. Whisk in the vanilla (if using). Lightly whisk in the remaining cream to create an ethereal whipped cream. Keep the whipped cream refrigerated, covered with plastic wrap, for up to 2 hours before using.