Place the lemon rind and juice in a small saucepan with 75 g (3 oz) of the caster sugar and 150 ml (1/4 pint) water. Bring to the boil. Meanwhile, blend 3 tablespoons water into the cornflour in a heatproof bowl, pour over the hot liquid, stirring constantly, and mix well until thickened. Stir in the egg yolk. Return to the pan and cook, stirring constantly, for 1 minute until thickened. Divide between 4 individual ramekins or gratin dishes.
Whisk the egg whites in a grease-free bowl until stiff. Add 1 tablespoon of the remaining caster sugar at a time, whisking well between each addition, until the meringue is smooth and glossy.
Spoon the meringue mixture over the lemon cups and sprinkle with the brown sugar. Place the dishes on a baking sheet and cook under a preheated high grill for 1–2 minutes until the tops are lightly golden and have firmed a little. Serve warm.