Start by whisking the egg whites in an electric mixer until they form satiny peaks. Add the caster sugar one tablespoon at a time until stiff and shiny.
Add cornflour and vinegar or lemon juice. Fold everything gently together.
Dab the four corners of a square sheet of greaseproof paper with the meringue mixture left on the whisk; this will act as a glue to stop the paper moving. Turn the paper over and spoon the remaining meringue on top.
Flatten and smooth into a circular shape.
