1. Preheat the oven to 140°C (120° fan) and line a large baking tray with paper.
2. Whisk the egg whites until they form stiff peaks. Gradually whisk in the sugar a spoonful at a time, then continue to whisk until the mixture is very shiny. Stir the cornflour into the vinegar, then whisk it into the meringue, stopping as soon as it is fully incorporated.
3. Spoon the meringue onto the prepared baking tray and spread it into a circle with a palette knife.
4. Bake the meringue for 1 hour 30 minutes or until it lifts easily from the greaseproof paper. Turn off the oven and leave the meringue to cool completely inside. This stage can be done a day in advance if you keep the cooled meringue in an airtight tin.
5. Whip the cream with the vanilla extract until it just holds its shape. Transfer the meringue to a serving plate and spoon the cream on top. Decorate with mixed berries and mint sprigs and serve soon after.