Preheat the oven to 200°C, gas mark 6. Line a 20cm square tin with baking parchment. Put the pumpkin for the filling into a baking tray with 2 tsp oil. Roast for 20 minutes until cooked through and soft then leave to cool.
Soak the cashew nuts for the pumpkin filling in a bowl of boiling water.
Add the nuts for the base to a food processor and blitz until the mixture resembles breadcrumbs. Add the rest of the ingredients for the base and blend until it comes together.
Spread this over the bottom of the prepared tin and compact it down with the back of a spoon so it holds together. Pop this into the fridge to harden while you make the pumpkin filling.
Drain and rinse the soaked cashews, then transfer to a clean food processor along with the rest of the pumpkin filling ingredients and a pinch of salt. Blend until really smooth.
This should take about 5 minutes. When it is ready, spread this mixture evenly on top of the nutty base layer. Put back in the fridge for about 1 hour to set.
When the cheesecake is firm, chop the dark chocolate into thin shards using a sharp knife on a chopping board. Sprinkle on top of the cheesecake to decorate before serving.