Heat oven to 180C/fan 160C/gas 4. Line the base of a 23cm springform tin with baking parchment. Whizz the biscuits in a food processor until they resemble breadcrumbs. Pour in the butter, whizz briefly again, then tip the mixture into the tin and press down firmly. Bake for 10 mins, then cool.
Reduce oven temperature to 140C/fan 120C/gas 1. Wrap a double layer of foil, lined with cling film, around the base and sides of the tin to make it watertight. Break 400g chocolate into a pan, then pour in the cream and heat until the chocolate has melted. Leave to cool slightly. Rinse the bowl of the processor, then blend the cream cheese, eggs and vanilla essence together with the melted chocolate and cream until smooth.
Put the tin in a deep roasting tray, then pour in the filling. Half-fill the roasting tray with boiling water, bake for 1 hr, then turn the oven off. Leave to cool in the oven with the door slightly ajar for 1-2 hrs. Lift the cheesecake from the water, discard the foil, then cover with cling film and chill. The cheesecake can be made up to 3 days ahead. Grate the remaining chocolate over the cheesecake before serving.
For the compote, heat oven to 180C/fan 160C/gas 4. Mix the rhubarb and sugar in an ovenproof dish. Scrape the seeds from the vanilla pod on top of the rhubarb, then drop in the pod, too. Pour the muscat over, cover with foil, then bake for 30-40 mins, stirring halfway, until the rhubarb is cooked. Leave to cool, then remove the pod. Serve the compote with the cheesecake. The compote can be kept in the fridge for up to 4 days.