In a large metal, ceramic or glass bowl (not plastic), beat the egg whites until soft peaks form.
Continue beating while adding the sugar a 1/4 cup at a time. The mixture should get glossier and thicker with each addition, and this should take at least 10 minutes. Beat in the vanilla, vinegar, and cornflour.
Spoon the mixture out onto the prepared tray, and shape into a dinner plate-sized mound.
Bake for approximately 1 ½ hours, until dry and crisp and it lifts easily off the baking paper. Turn the oven off and leave the pavlova for at least an hour before removing it from the oven. Finish cooling on a wire rack.
When completely cool, place on a serving plate, swirl the top with the whipped cream and decorate with sliced or chopped fruit of your choice.
Older eggs work better as the white breaks down easier.
Wipe the bowl beforehand with lemon juice to remove any greasy residue before beating the egg whites (grease affects the ability for the egg whites to form peaks).
Caster sugar is needed as the fine crystal size traps and holds air - more air is trapped because there are more crystals.