Put the chocolate chip cookie crumbs in a bowl, add the cinnamon and melted butter, mix well and tip into a 22cm push-up-bottom or springform tin. Spread the mixture out evenly. Chill in the fridge until hard and set.
Melt the dark chocolate in a bain-marie (in a heatproof bowl over a pan of simmering water). Stir in the Kahlua or Tia Maria.
Grate the white chocolate. Cut the vanilla pod in half lengthways, scrape out the seeds and add to the grated white chocolate.
Beat together the creme fraiche and demerara sugar until blended, then add the double cream and mix until smooth. Divide the mixture into 2 bowls. Add the melted dark chocolate and liqueur to the mixed creams in one bowl, and add the grated white chocolate and vanilla seeds to the other.
Spoon the two chocolate mixtures on to the chilled biscuit base and lightly mix with a fork to obtain a marbled effect. Chill for 3 hours before serving.