Preheat the oven to 170C/150C Fan/Gas 3. Grease and line the base and sides of a 13cm/5in, a 18cm/7in and a 23cm/9in deep, loose-bottom cake tin with non-stick baking paper.
Melt the butter and chocolate in a saucepan over a low heat. Stir in the crushed biscuits and divide between the 3 tins. Press evenly over the base of the tins and chill in the fridge while you make the filling. (Make sure the base covers right to the edge, or the topping could leak through the join on the loose bottom.)
For the filling, break the white chocolate into a bowl and melt very gently over a pan of hot water (do not allow the chocolate to become too hot), stir occasionally with a spoon until runny and smooth.
Beat the cream cheese and eggs together in a large bowl until smooth, add the soured cream and vanilla and whisk again until completely smooth with no lumps. Stir in the melted chocolate and mix together.
Divide half of the mixture between the 3 chilled bases. Scatter the raspberries over the mixture in each tin and cover with the remaining mixture, spreading evenly.
Bake in the preheated oven for 30-35 minutes for the 13cm/5in cheesecake, 35-40 minutes for the 18cm/7in cheesecake and 40-45 minutes for the 23cm/9in, until they are just firm around the edge and just set in the middle. Remove from the oven, and then leave to cool at room temperature before transferring to the fridge to chill.
For the decoration, pulse 50g/1Â¾oz of the raspberries with the icing sugar in a food processor to make a purÃ©e. Pass through a sieve and discard the seeds.
Turn the cheesecakes out of their tins and place the 23cm/9in cheesecake on a serving plate. Place the 18cm/7in and 13cm/5in cheesecakes on 3mm-thick cake boards. Cut a cake dowel or an invisible cake pillar to the same height as the largest cheesecake. Press down into the middle of the cheesecake and place the 18cm/7in cheesecake on top. Repeat the process with the 18cm/7in cheesecake and place the 13cm/5in cheesecake on top. The dowel or invisible pillar helps to support the cheesecakes and prevents the lower layers from being crushed.
Whip the cream until soft peaks form when the whisk is removed. Spoon into a piping bag fitted with a medium star nozzle. Pipe rosettes of cream close together around the top edge of all 3 cheesecakes and top each rosette of cream with a raspberry. Pour the raspberry coulis on to the top tier - the piped cream should prevent it from dripping down the sides.
To make white chocolate curls, place the chocolate bar on its side and drag the blade of a long sharp knife at a slight angle towards you (for easier, but shorter, curls use a vegetable peeler). Arrange the curls over the top tier.