Preheat oven to 150°C. Line a large baking tray with non-stick baking paper.
Place the egg whites in the bowl of an electric mixer and whisk on high speed until stiff peaks form. Gradually add the sugar, 1 tablespoon at a time, waiting 30 seconds between each tablespoon. Whisk for 6 minutes or until stiff and glossy.
Scrape the sides of the electric mixer, add the vinegar and whisk for 2 minutes or until glossy and combined.
Spoon 6 rounds onto the baking tray. Reduce the oven temperature to 120°C and bake for 30 minutes or until crisp to touch.