Bring 1 cup of whey to the boil.

Mix the sugar and cornflour together and add to the remaining cold whey to make a smooth paste.

Combine with the hot whey and cook until thick, stirring constantly.

Combine this thickened mixture with lightly beaten egg yolks, butter, salt, lemon juice and zest.

Cook for two minutes then pour into the pie shell.

Whisk the egg whites with the cream of tartar until light and frothy.

Add the sugar a little at a time, and whisk until the mixture forms stiff peaks.

Top the pie with the meringue and bake in a moderate oven 350 deg F/180 C/gas mark 4 for 12 to 15 minutes.