Preheat oven to 150℃ (130℃ fan forced). Line a baking tray with baking paper. Using a lead pencil, draw a 20cm round circle then flip the paper over, pencil side down.
Place egg whites into a clean bowl of a stand mixer. Whip using the wire whisk attachment on medium speed (I used Speed 6) for about 1 minute or until soft peaks form.
Add all the caster sugar at once then beat for 12-15 minutes on Medium-High Speed (I used speed 8 on my KitchenAid) or until the mixture is smooth and super glossy. To test if its ready, grab a little of the mixture between 2 fingers and rub together. If it feels 'grainy' at all, continue beating until you can't feel any of the sugar. It needs to be fully dissolved.
Once mixture is smooth with no grainy granules, sprinkle over the cornflour and vinegar. Continue beating on medium speed for around 20-30 seconds.
Spoon mixture onto baking tray within the circle you drew. Using a spatula or spoon, make a neat circle of mixture and smooth the top and sides creating a little crater in the top surface.
Place into preheated oven. Reduce oven temperature to 130℃ (110℃ fan forced) and bake for 1 hour 15 minutes. Turn oven off and leave to cool completely in the oven with the door closed.