1. Prepare the Crust:
Start by melting the butter in the microwave and allow it to cool slightly.
Make the biscuit and hazelnut crumbs using a food processor, or by placing them in a zip bag and crushing them with a rolling pin.
Mix the crumbs with the warm (but not hot) melted butter and a pinch of salt until the mixture comes together.
Line a 20 cm (8-inch) loose-base pie tin with the crumb mixture, pressing it evenly and tightly into the base and sides. If the mixture is too runny, refrigerate it for a few minutes before pressing it into the tin.
Once the crust is evenly distributed, place it in the freezer while you prepare the filling.
2. Make the Chocolate Hazelnut Ganache Filling:
Heat the heavy cream until it is just simmering, then pour it over the chocolate and hazelnut butter mixture.
Stir continuously or use a hand blender to combine until the mixture is smooth and fully incorporated.
Pour the chocolate hazelnut ganache into the prepared crust, smoothing the top with an offset spatula.
Place the tart in the refrigerator and allow it to set for at least 4 hours.
3. Decorate and Serve:
Once the tart has set, carefully remove it from the tin.
Decorate the top with roasted hazelnuts before serving.