Heat the oven to 180C/fan 160C/gas 4. crush the biscuits in a food processor (or put them in a plastic bag and bash with a rolling pin). mix with the butter. Press into a 20cm spring-form tin and bake for 5 minutes. cool.
Beat the cream cheese with the flour, sugar, a few drops of vanilla, eggs, the yolk and soured cream until light and fluffy. Stir in half the raspberries and the white chocolate and pour into the tin. Bake for 40 minutes and then check - it should be set but slightly wobbly in the centre. Leave in the tin to cool completely, then chill for 2 hours.
Just before serving, put most of the raspberries in a pan with 1 tbsp icing sugar (keep a few aside). Heat until juicy and then squash with a fork. Push through a sieve. Mix with the framboise (if using) and the whole raspberries then serve with the cheesecake.