To make the pastry, sift the flour, cocoa powder, icing sugar and salt into a bowl. Gently melt the butter with the water and stir into the flour mixture with a wooden spoon to make a smooth dough. Press the warm dough evenly over the base and up the sides of a 26-27.5 cm/10½-11 level in loose-based fluted flan tin and then chill while you prepare the filling.
Separate 4 of the eggs and put the egg yolks, the 2 whole eggs and the lemon rind into the top of a double boiler or a basin. Whisk together thoroughly with a fork. Then mix in 150 g/5 oz of the caster sugar. Put the lemon juice into a measuring jug and bring up to 300 ml/½ pint with water. Stir into the egg yolk mixture. Put over gently simmering water and stir until the mixture thickens enough to coat the back of the spoon lightly. Now add the unsalted butter bit by bit and stir until melted. Remove from the heat and leave until cool.
Preheat the oven to 190°C/375°F/gas mark 5. Pour the cooled lemon curd into the chilled chocolate case and cook in the centre of the oven for 20-25 minutes. Remove from the oven and leave to cool for 10 minutes. Turn the oven down to 180°C/350°F/gas mark 4. When the tart has cooled slightly, put the egg whites into a large bowl, add the salt and whisk until the whites stand in soft peaks. Whisk in about half the remaining sugar, then fold in all the remaining sugar with a metal spoon. Spoon the meringue on top of the pudding and sprinkle with the chopped chocolate. Cook in the centre of the oven for 10-15 minutes, until the meringue is pale gold and the chocolate has melted. Remove from the oven and leave in the tin for about 10 minutes, then push the base up out of the rim and place on a serving plate. Serve warm or cold, but eat on the same day.