Preheat the oven to 170C/150C Fan/Gas 3. Grease the base of a 20cm/8in springform cake tin with butter, then line with a circle of baking parchment.
For the base, melt the butter and chocolate in small saucepan over a low heat. Stir in the crushed biscuits.
Transfer the base mixture to the cake tin and, using your fingers, press down until it completely covers the bottom of the cake tin in an even layer. Chill in the fridge until needed.
For the filling, suspend a heatproof bowl over a pan of gently simmering water (do not allow the base of the bowl to touch the water). Add the white chocolate to the bowl and melt until runny and smooth, stirring occasionally with a wooden spoon.
In a large bowl, whisk the cream cheese and soured cream together until smooth and well combined. Whisk in the eggs and vanilla extract until completely smooth, then stir in the melted white chocolate until well combined. Fold in the chopped ginger.
Pour the filling mixture on top of the chilled base and level the top using a palette knife. Bake in the oven for 40-45 minutes, or until firm around the edge and just set in the middle.
Remove the cheesecake from the oven and rest for 10 minutes, then run a small blunt knife around the inside edge of the cake tin. Set aside until the cheesecake has completely cooled in the tin, then chill in the fridge.
For the decoration, lay two strips of baking parchment on a work surface. Spoon the melted chocolate into a small piping bag fitted with a size-two writing nozzle and pipe eleven small chocolate holly leaf shapes onto the baking parchment strips (five on one, six on the other).
Lay the strips of baking parchment paper over two rolling pins or similar slightly curved surfaces. Secure the strips at each end with elastic bands. Place the rolling pins onto the bases of two empty egg boxes to prevent them rolling away, then allow the chocolate leaves to cool and set.
Meanwhile, add a few drops of red food colouring to the marzipan and knead until the marzipan is a uniform bright red. Dust your hands with icing sugar, then roll the marzipan into 27 small balls (these will be berries for the chocolate holly leaves).
Remove the cake-tin ring from the cheesecake and place the cheesecake, still on its base, onto a serving plate.
Whip the cream until soft peaks form when the whisk is removed, then spoon into a piping bag fitted with a star nozzle. Pipe eight rosettes or swirls around the edge of the top of the cheesecake and one rosette or swirl in the middle.
Place one chocolate holly leaf and three holly berries in the centre of each swirl of cream, then arrange three chocolate holly leaves and the remaining marzipan berries on top of the central swirl of cream. Serve immediately.