1. Preheat the oven to 160°C and line a baking tray with baking paper. Using a pencil, draw a circle 23cm in diameter on the back of the paper.
2. Using an electric beater, whisk the egg whites, cream of tartar and salt in a large mixing bowl until soft peaks form. Now add the sugar gradually – at most, one tablespoon at a time. By the time all the castor sugar has been added, it should be thick and glossy. Finally, add the lemon juice, cornflour and vanilla until just blended.
3. Using a spoon or piping bag, form a meringue circle on the prepared baking tray. Now build up the outer edge with dollops of meringue so that the sides are high enough to hold the filling. Place in the oven and immediately reduce the temperature to 120°C; bake the meringue for 1 hour and 15 minutes. Switch off the oven and allow the meringue to cool in the oven.