Preheat oven to 350°F (180°C). Line a baking sheet with parchment and trace a 9-inch circle. Wipe your bowl, whisk, and spatula with vinegar to remove any grease.
Score peaches, blanch in boiling water for 30 seconds, then transfer to ice water. Peel, slice into wedges, and toss with 1 tablespoon sugar and 1 teaspoon lemon juice. Refrigerate for 30 minutes.
Separate cold eggs and let whites come to room temperature. Beat on medium speed until foamy, then continue until soft peaks form where the peaks curl over.
With mixer running, add superfine sugar one tablespoon at a time over 5-7 minutes. Whip until meringue is thick, glossy, and holds stiff, upright peaks with no grittiness.
Remove bowl from mixer. Sprinkle cornstarch, vinegar, and vanilla over meringue and fold gently with a figure-eight motion until just combined.
Spoon meringue onto the parchment within the traced circle. Use an offset spatula to spread it out, building up the edges slightly so the center is lower than the rim.
Place in oven and immediately reduce temperature to 225°F (110°C). Bake for 1 hour 15 minutes to 1 hour 30 minutes until dry and firm on the outside with a slight give in the center.
Turn off the oven and leave pavlova inside with door closed for at least 1 hour, or ideally overnight, to cool gradually and prevent cracking.
Just before serving, whip heavy cream with powdered sugar until it holds soft, billowy peaks. Be careful not to over-whip.
Carefully transfer cooled meringue to a serving plate. Spoon whipped cream into the hollow center and top with macerated peaches. Drizzle with accumulated peach syrup and add optional garnishes. Serve immediately.