Step 1: Prepare the Pan

1. Flip the base of an 8-inch springform pan upside down.
2. Butter the base, line it with baking paper, and clip the sides onto the base with excess paper overhanging.
3. Butter and line the sides of the pan with more baking paper.

Step 2: Make the Oreo Base

1. Blitz Oreo cookies in a food processor until fine crumbs form.
2. Add melted butter to the crumbs and blitz until combined.
3. Transfer the mixture to the prepared pan and press it evenly with the back of a flat cup. Chill in the fridge.

Step 3: Prepare the Nutella Mousse Filling

1. Sprinkle gelatine over water in a small bowl, stir, and let bloom for 5 minutes. Microwave for 15 seconds to liquefy, then cool for 5 minutes.
2. Beat cream cheese, Nutella, and icing sugar with an electric mixer until smooth. Mix in the gelatine until combined.
3. In a separate bowl, whip the heavy cream until stiff peaks form.
4. Fold a third of the whipped cream into the Nutella mixture, then gently fold in the rest in two additions.
5. Pour the filling over the chilled Oreo base and smooth the top. Refrigerate for at least 1 hour.

Step 4: Make the Nutella Ganache Topping

1. Combine cream, Nutella, and chocolate chips in a heatproof bowl.
2. Microwave in two 30-second bursts, stirring in between until smooth. Let cool for 5 minutes.
3. Pour the ganache over the chilled mousse layer and spread evenly. Chill for 3 hours or until firm.

Step 5: Decorate the Cheesecake

1. Sprinkle toasted hazelnuts over the set ganache.
2. Pipe whipped cream around the edges of the cheesecake.
3. Drizzle warmed Nutella over the top.
4. Slice with a hot knife (clean between slices) and serve.