Lightly grease a 9in/23cm loose-bottomed cake tin and line the base and sides with greaseproof paper. Beat together all the ingredients for the base and press over the bottom of the tin. Chill for 10- 15 minutes while you make the cheesecake mixture.
Put the butter, sugar, cheese, cornflour, vanilla extract, lemon rind and egg yolks into a large bowl and beat together until completely smooth, then fold in the soured cream.
Whisk the egg whites until stiff then fold into the mixture using a large metal spoon. Pour on to the chilled base and bake at 325F/160C/gas mark 3 for about 1 hour or until lightly set.
Turn off the oven, leaving the cheesecake inside for a further hour to cool. Then open the oven door and allow the cheesecake to stand and cool completely for a further hour. Chill and serve in wedges.