White chocolate and lemon roulade

4 lemons

231g of caster sugar

4 eggs, whisked together with 1 egg yolk

221g of white chocolate, good quality

3 duck eggs, separated

85g of icing sugar, sifted

25g of cornflour

25g of soft flour, for pastry

pistachio nuts, green

250g of unsalted butter, diced

330g of double cream

30g of semi-skimmed milk

butter, for greasing

2g of gelatine, approximately