Line the bottom of a 1 1/4l/2 pint (1 1/2 quart) mould with a circle of greaseproof or waxed paper.
Set aside.
Mix together 75ml/3floz (3/8 cup) each of the milk and water.
One by one, dip the biscuits (cookies) into the mixture, coating them well, but being careful not to saturate them.
Line the sides of the mould with the biscuits (cookies), reserving the extra to use later.
Trim the biscuits (cookies) to fit the top of the mould.
Set aside.
Dissolve the sugar in the remaining milk, stirring constantly until it has dissolved.
Stir in the dissolved gelatine, then the chestnut puree and orange-flavoured liqueur.
Remove the pan from the heat and transfer the mixture to a bowl.
Chill in the refrigerator for 30 minutes, or until the mixture is beginning to set.
Drain the preserved chestnuts, reserving the syrup.
Chop coarsely and set aside.
Beat the cream until it is thick but not stiff.
Beat half the cream into the gelatine mixture, then fold in the remaining cream.
Fold in the chopped chestnuts.
Spoon the mixture into the lined mould.
Arrange the remaining biscuits (cookies) over the top, to cover the mould completely, and trim off any protruding parts.
Cover with greaseproof or waxed paper and chill in the refrigerator for at least 6 hours or overnight.