Prepare the Crust:

1. Preheat the Oven: Preheat your oven to 325°F (163°C) and grease a 9-inch springform pan with butter or cooking spray.
2. Make the Crust: In a bowl, mix graham cracker crumbs, crushed hazelnuts, melted butter, and sugar until the mixture is well combined and slightly damp.
3. Press the Crust: Press the crumb mixture evenly into the bottom of the prepared pan to form a firm crust. Use the back of a spoon to compact it.
4. Bake the Crust: Bake in the preheated oven for 10 minutes, then remove it and allow it to cool while you prepare the filling.

Prepare the Filling:

5. Beat the Cream Cheese: In a large mixing bowl, beat the softened cream cheese and sugar with an electric mixer until smooth and creamy.
6. Add Vanilla and Hazelnut Spread: Mix in the vanilla extract and hazelnut spread until fully combined.
7. Add Eggs: Add the eggs one at a time, beating well after each addition.
8. Add Sour Cream and Heavy Cream: Mix in the sour cream and heavy cream until smooth. The filling should be thick but pourable.
9. Pour the Filling into the Pan: Pour the cheesecake filling over the cooled crust in the springform pan, spreading it evenly.

Bake the Cheesecake:

10. Bake the Cheesecake: Bake at 325°F (163°C) for 50-60 minutes or until the center is almost set (it should still slightly wobble when shaken).
11. Cool the Cheesecake: Turn off the oven, crack the oven door, and let the cheesecake cool for about 1 hour. Then, transfer it to the fridge and chill for at least 3 hours, or overnight for the best results.

Prepare the Ganache:

12. Make the Ganache: In a small saucepan, heat the heavy cream and butter over medium heat until it begins to simmer. Remove from heat and stir in the dark chocolate until smooth and glossy.
13. Cool the Ganache: Let the ganache cool for 10 minutes before pouring it over the chilled cheesecake.

Garnish and Serve:

14. Top the Cheesecake: Pour the ganache over the chilled cheesecake, allowing it to gently drip down the sides. Garnish with chopped hazelnuts and whipped cream if desired.
15. Chill and Serve: Let the cheesecake set in the fridge for another 30 minutes before slicing and serving.