1) Soak the leaf gelatine in a bowl of cold water to cover. When the sheets are floppy, pour off the water and set aside.
2) In a medium heatproof bowl mix together 30g sugar, the cornflour and egg yolks to a creamy paste. Set aside.
3) Put the lemon zest and juice into a saucepan with 100ml water and bring to the boil. Then whisk in the cream and return to the boil.
4) Pour the boiling liquid onto the sugar-cornflour paste, whisking as you pour until blended then return the mixture to the pan and over a low heat, stir with a wooden spoon until thickened and smooth.
5) Remove from the heat and stir in the soaked gelatine sheets until dissolved. Scoop into a larger bowl then cover and leave to cool. (In the restaurant we would place the bowl in bigger bowl of ice to speed the cooling, stirring often).
6) When the mixture is cool, whisk the egg whites to the soft peak stage (you could use an electric whisk for this) and then gradually whisk in the remaining sugar to make a thick meringue. Fold into the lemon cream using a large metal spoon until well blended.
7) Line a 1kg non-stick loaf tin with baking parchment and spoon in the mixture, chill until set then freeze until solid. To serve, dip the frozen loaf tin quickly in a bowl of hot water and loosen the sides with a knife then upend and shake out. Cut into 6 to 8 slices with a serrated knife.
8) If you wish to caramelise the slices then wave a heated cooks' blow torch on top before serving.