In a big bowl place the flour, salt, butter and the zest of 1 of the lemons, integrating everything together with your hands.
Add the cold water, make a dough and leave it to rest in the fridge for 30 minutes.
Stretch the dough using your fingers to cover the dish.
Prick the surface with a fork and bake for about 30 minutes.
In a deep pan, mix the cornflour, sugar, zest of the other lemon, boiling water and lemon juice.
Cook on low heat, stirring continuously, until boiling point.
Remove from the heat, and leave to cool down a bit.
Add the beaten yolks and the butter.
Stir until obtaining a smooth cream.
Pour the cream on top of the pre-baked base.
Get the oven temperature to maximum or use the oven´s grill.
Place the egg whites and sugar in a Bain Marie (place a pan with water to boil and on top the bowl with the sugar and whites).
Stir the sugar in the whites with a spoon until dissolved. Remove from heat.
Beat with an electric mixer until cold and hard.
Cover the lemon cream with the meringue.
Bring the pie to the top shelf of the oven or grill, observing it all the time, until nicely golden. Be extra careful not to burn it! It only takes very few minutes.
Let the pie to cool down and bring to fridge until time for serving.
Tip: if you prefer a taller meringue layer, use 8 egg whites instead of 4.