Place the crumbled chocolate cantuccini biscuits in a serving bowl, reserving a few pieces for decoration Drizzle over the sherry, then spread over the jam.
Pour the milk into a pan. Cut a slit down the length of the vanilla pod and scrape the seeds into the pan with the pod. Bring to the boil. Turn off the heat and leave to cool for 5 minutes.
Whisk the egg yolks, sugar and cornflour together in a bowl. Remove the vanilla pod, then whisk the warm milk into the egg mixture. Pour the custard back into the pan. Heat gently, whisking continuously, for 2–3 minutes or until the mixture thickens. Pour the custard over the jam layer. Cool and chill until firm.
Whip the cream until softly peaking, fold together with the crème fraîche, vanilla extract and sugar. Spread over the top of the custard, then dollop on a few extra spoonfuls to decorate. Sprinkle with the reserved crumbled cantuccini biscuits and the dragees.