Preheat the oven to 120C/250F. Line a large baking tray with tin foil and lightly grease.
In a large mixing bowl, beat you egg whites until stiff peaks form. Using a tablespoon, add the superfine sweetener, until mixed through and glossy. Add the cornstarch, vinegar, and vanilla extract until combined.
Transfer the meringue mixture onto the foil lined sheet. Push together into a circular shape.
Bake the meringue for 90 minutes, or until golden around the sides and tender. Turn the oven off and keep the meringue in there to cool completely. Be sure to leave the door slightly ajar.
Once the meringue has cooled completely, prepare the whipped topping. Beat together the heavy cream and powdered sweetener until thick and smooth. Gently spread over the top of the meringue. Decorate with berries of choice and refrigerate for at least an hour, to set.
* To make a superfine sweetener (caster sweetener), place 1 1/2 cups erythritol or monk fruit sweetener in a blender. Blend until a fine texture. You will need 1 1/4 cups of the blended amount of sweetener. TO STORE: If you've layered and decorated your pavlova, you can store it in the refrigerator, covered, for up to 3 days. However, if you are storing the meringue base, you can refrigerate it for up to 2 weeks. Be sure that it is completely covered (plastic or glad wrap works very well) to ensure that no air pockets can seep through. TO FREEZE: Only the meringue portion can be frozen. Cover it completely in plastic wrap, and store in the freezer for up to 2 months. When ready to use, refrigerate it until it is no longer frozen.