Prepare the pan
1. Flip the base of an 8-inch (20 cm) springform pan upside down, so it’s easier to remove the finished cheesecake without the lip in the way.
2. Butter the pan base, then press on a square sheet of baking paper. Clip the pan sides onto the base, letting the excess paper overhang. Butter and line the pan sides with more baking paper.
Make the Oreo base
3. Roughly break up the Oreos with your hands and place them in a food processor. Blitz until fine crumbs. Add the melted butter and blitz again until combined.
4. Transfer the crumbs to the prepared pan, pressing evenly and firmly onto the base (the underside of a straight-sided, flat-bottomed cup measure works well).
Make the Nutella mousse filling
5. Place the water in a small bowl and sprinkle the gelatine powder across the surface. Stir to partly dissolve and set aside for 5 minutes, until thick (this is called blooming).
6. Microwave the gelatine for 15 seconds to turn it liquid, stir, then let it stand for 5 minutes to cool.
7. Beat the cream cheese, Nutella, and powdered sugar until smooth. Add the cooled gelatine liquid and beat for another 30 seconds.
8. In a separate bowl, whip the cream until stiff peaks form.
9. Add one third of the whipped cream to the Nutella mixture and gently fold until just combined. Fold in another third the same way, then fold in the remaining cream.
10. Pour the mixture into the prepared pan and refrigerate for at least 1 hour to set.
Make the ganache and decorate
11. Place the cream, Nutella, and chocolate chips in a heatproof bowl. Microwave in two 30-second bursts, stirring in between, until smooth.
12. Let the ganache cool for 5 minutes, then pour it over the set cheesecake. Tilt the pan to spread it evenly over the surface.
13. Refrigerate the cheesecake for at least 3 hours before adding the hazelnuts (otherwise they sink into the cake).
14. Whip the 1/2 cup heavy cream until stiff peaks form. Sprinkle the chopped hazelnuts over the cheesecake, pipe dollops of whipped cream around the edge, then drizzle with the warm Nutella. Slice into wedges and serve.