Preheat the oven to 300°F (150°C). Line a large baking tray with parchment paper.
Place the egg whites in a clean, grease-free mixing bowl. Beat on medium speed until foamy, then increase the speed and continue beating until soft peaks form. Add the cream of tartar now if using.
Add the caster sugar one tablespoon at a time, beating well after each addition. Continue whipping until the meringue is thick, glossy, and holds stiff peaks.
Gently fold in the cornstarch, vinegar or lemon juice, and vanilla extract. Do not overmix.
Spoon the meringue onto the prepared tray and shape it into an 11–12 inch circle. Make the center slightly lower than the edges to create room for the topping.
Place the pavlova in the oven and immediately reduce the temperature to 250°F (120°C). Bake for 1 hour 45 minutes to 2 hours.
Turn off the oven and leave the pavlova inside until completely cool. Slow cooling helps reduce cracking and collapsing.
Whip the heavy cream until soft peaks form. Add powdered sugar if desired.
Spoon the whipped cream over the cooled pavlova and top with fresh fruit just before serving.