Position a rack in the center of the oven and preheat to 250°F (120°C). Line a baking sheet with parchment paper and trace a 9-inch (23cm) circle on the parchment, then flip it over so the pencil side faces down.
Ensure the bowl and whisk are spotless and dry. Add the room-temperature egg whites and a pinch of salt, then beat on medium speed until foamy and holding soft peaks.
With the mixer running on medium, add the superfine sugar one tablespoon at a time, waiting about 20 seconds between additions, until the meringue is thick, glossy, and holds stiff peaks. Rub a little between your fingers; if gritty, keep beating until smooth.
Sprinkle the cornstarch over the meringue, add the vinegar and vanilla, and beat on low for just a few seconds until combined.
Scoop the meringue onto the traced circle and spread it with an offset spatula, building the sides slightly higher than the middle. Create a shallow well in the center and run the spatula up the sides to make decorative peaks.
Place the tray in the oven and immediately reduce the temperature to 225°F (110°C). Bake for 1 hour 15 minutes without opening the door, until the outside is dry, pale, and lifts cleanly from the parchment.
Turn the oven off and prop the door open a couple of inches. Leave the pavlova inside to cool gradually for at least 1 hour 30 minutes, or until completely cool.
Pour the cold heavy cream into a chilled bowl, add the powdered sugar and vanilla, and beat until the cream holds soft, spreadable peaks.
Slice the peaches into wedges and toss gently with lemon juice. If using the glaze, warm the peach jam until pourable.
Transfer the cooled meringue to a serving plate. Spoon the whipped cream into the center well, pile the peach slices over the cream, drizzle with warmed peach jam, scatter mint leaves on top, and serve right away.