Grease 4 x 15cm springform cake tins and wrap the bases and sides tightly in foil.
Combine the melted butter and ginger biscuits, divide among the tins and flatten to cover the bases. Transfer to the fridge to chill for an hour.
Preheat the oven to 140°C, gas mark 1. Put the cream cheese, sour cream, caster sugar, cinnamon, nutmeg, vanilla paste and cornflour in a bowl. Beat with an electric hand whisk to combine. Gradually add the eggs and continue to beat until combined. Spoon the filling over the cheesecake bases and spread evenly.
Divide the cheesecakes between 2 deep roasting trays, then pour in just enough boiling water to reach halfway up the sides of the tins.
Bake for 45 minutes, until well risen and just set, with a slight wobble in the centre. Leave to cool with the oven door slightly open for an hour, then cool to room temperature before transferring to the fridge to set overnight.
Carefully remove the cheesecakes from their tins. Scatter the granulated sugar over the top of one cheesecake, shake over 1 tsp water using your fingers and caramelise under a hot grill or with a blowtorch, until golden and crunchy.
Stack the cheesecakes onto a cake stand or platter, finishing with the brûléed cheesecake on top. Decorate with a garland and baubles, if you like.