Preheat your oven to 300 degrees F. Spray a 9 inch tart pan with non stick spray and set aside.
Using a food processor, pulse your hazelnuts, flour, sugar, and salt together until it reaches the consistency of coarse meal. (Reserve 1/4 cup of the nuts for topping.) Add the cubes of cold butter and pulse until the mixture turns into small crumbs.
Transfer crust mixture into a large bowl. Using a rubber spatula, slowly mix in 3 tbsp of ice water. Tightly press the dough evenly into your tart pan.
Bake for 18-20 minutes. Remove from the oven and place on a wire rack so it can slightly cool as you prepare the filling. Turn off your oven.
Start here is using a store bought crust:
Make your filling by whisking your cornstarch and ½ cup of heavy cream in a small bowl. To prevent clumping ensure you are constantly whisking as you add the cornstarch.
Add your heavy cream, vanilla extract, Nutella, and salt into a medium saucepan. Cook over medium heat, whisking constantly until the mixture is well combined and smooth.
Add the cornstarch and cream mixture, whisking constantly to avoid lumps in the filling. Let the mixture thicken up for 2-3 minutes whisking constantly. Remove from your heat and continue whisking for 2 more minutes.
Evenly pour your Nutella filling into your crust and smooth.
Chill your tart in the fridge for at least 3 hours to let it set.