Cover the bottom of a 22cm springform tin with greaseproof paper.
Melt 50g of the butter, crush the biscuits and mix together. Press them evenly into the tin. Cream the remaining butter and the sugar together until light and fluffy. Beat in the egg yolks, lemon rind and juice and cream cheese. Whip the cream until it just holds its shape; whisk the egg whites until stiff and fold both i