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 volumn 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 cornstarch.