For the crust:
For the filling:
1. Toast raw hazelnuts in a pan for 3-4 minutes. Wrap the hazelnuts in a kitchen towel and rub vigorously to remove as much loose skin as possible. Roughly chop and set aside.
2. In a food processor or a Ziploc bag crush the cookies into fine crumbs. Add melted butter and pulse until combined. Press the mixture into the bottom and up the sides of an 9-inch (23cm) tart pan with a loose base. Refrigerate while preparing the filling.
3. In a small bowl mix 1/4 cup (60ml) heavy cream and cornstarch. Set aside.
4. In a medium sized saucepan combine Nutella, salt and heavy cream. Place over low-medium heat until melted and combined. When Nutella is melted and the mixture is simmering add cornstrach mixture, cook over medium heat, stirring constantly until thickened. About 5-6 minutes.
5. Pour the filling over the crust. Decorate with chopped hazelnuts.
6. Refrigerate for at least 4 hours or until set.