Prep: 30 mins | Cook: 1 hour
Pre-heat oven to 150 C / Gas mark 2. Line a baking tray with baking parchment. Draw a 23cm (9 in) circle on the parchment.
In a large bowl, beat egg whites until stiff but not dry. Gradually add in the sugar, 1 tablespoon at a time, beating well after each addition. Beat until thick and glossy. Overbeaten egg whites lose volume and deflate when folded into other ingredients. Be absolutely sure not a particle of grease or egg yolk gets into the whites. Gently fold in vanilla extract, lemon juice and corn flour.
Spoon mixture inside the circle drawn on the parchment. Working from the centre, spread mixture towards the outside edge, building edge slightly. This should leave a slight depression in the centre.
Bake for 1 hour. Cool on a wire rack.
Remove the parchment, and place meringue on a flat serving plate. Fill the centre of the meringue with whipped cream, sweetened if desired