Pastry: Place the butter, flour, sugar and egg yolk in a food processor. Process on medium speed for about 20 seconds.
Roll out pastry to fit a 23 cm pie plate. There should be a fair amount of overhang (don`t trim it off). Leave in fridge for 20 minutes.
Fit a large sheet of foil into the pastry case, bringing it well up the sides. Fill with baking beans, rice or similar.
Bake the pastry 12-15 minutes at 200 °C. Remove the foil and beans. Return the pastry to oven for five more minutes. Trim the top of the pastry case with a very sharp knife.

Filling: Boil water, corn flour and sugar until it thickens. Add lemon zest and juice, egg yolks and butter. Bring to the boil again, while whisking constantly (to prevent it from curdling), until the mixture becomes pale in colour and quite thick. Allow to cool.

Meringue: In a clean bowl, beat the egg whites until foamy. Gradually add the sugar and continue to whip until stiff peaks form.

Spread filling in the pastry case and top off with the meringue. Bake in a 175 °C oven for about 15 minutes, or until the meringue achieves a lovely golden colour.