Preheat oven to 300F/150C
Get some paper towel, and soak it in in a little white vinegar. Wipe out your bowl and whisk with the vinegar.
Whisk together the egg whites and salt until stiff peaks form.
Gradually add sugar, ¼ of a cup at a time and whisk after each addition for about 1 minute. Whisk until sugar has dissolved. To test to see if it's dissolved, gently rub a little bit of the meringue mixture between your fingers. If it's grainy, keep whisking.
After the last of the sugar has been added, whisk on high for 3 minutes. The mixture should be very thick and glossy.
Combine the cornflour/cornstarch, vinegar and vanilla essence in a very small bowl.
Add to the meringue mixture and whisk until just combined.
Line a 12" round pizza tray with a piece of baking paper/parchment paper cut into a circle.
Start scooping out the meringue mixture, forming a circle roughly 8-9" in diameter.
Use a spatula, to gently start shaping your pavlova. Start at the bottom and gently make nice neat lines moving toward the top of the pavlova (see image).
Reduce oven temperature to 250F/120C and place pavlova in the oven on the middle shelf.
Cook for 1 hour to 1 hour and 15 minutes. Turn on oven light to see the pavlova. The pavlova should be lightly browned. If you to, quickly open the oven door and touch the pavlova. It should be dry to the touch.
Turn off the oven, and leave the pavlova in the oven with the door closed for 3 hours until the pavlova is cooled down.
To serve, whip the cream, and gently spoon into the centre of the pavlova. Top with fresh berries.