Crush the biscuits in a food processor, then turn them into a bowl. Gently melt the butter in a small saucepan, pour on to the biscuits and stir in. Press the mixture evenly over the bottom of a well-buttered 22-23 cm/8½-9 in springform cake tin and then chill.
Preheat the oven to 170°C/325°F/gas mark 3. Put the 2 cheeses into a food processor with the milk, cornflour, caster sugar and salt. Separate the eggs, putting the yolks in the food processor and the whites in a large bowl. Extract the cardamom seeds from the pods, grind them in a pestle and mortar or a coffee grinder and add to the food processor. Whizz thoroughly and turn into a bowl.
Add a pinch of salt to the egg whites, then whisk them to soft peaks and fold gently into the cheese mixture. Pour on to the chilled base. Bake in the centre of the oven for 50-60 minutes, until just firm to a soft touch in the centre. Leave to cool completely, then gently release the sides of the tin and, using a palette knife, carefully transfer the cheesecake from the base on to a serving plate. Before serving, sprinkle with icing sugar through a sieve.