Toast the Hazelnuts:
Preheat oven to 300°F (149°C). Spread chopped hazelnuts on a parchment-lined baking sheet. Toast for 5–7 minutes until fragrant. Cool for 5 minutes. Increase oven to 350°F (177°C).

Make the Crust:
In a food processor, pulse 3/4 cup of toasted hazelnuts, flour, sugar, and salt until coarse. Add cold butter and pulse into pea-sized crumbs. Transfer to a bowl and stir in ice water until the dough comes together.

Form and Bake the Crust:
Press dough firmly into a 9-inch tart pan. Line with foil/parchment and fill with pie weights. Bake for 18–20 minutes. Cool slightly.

Prepare the Filling:
In a small bowl, whisk cornstarch and 1/2 cup heavy cream until smooth. In a saucepan, combine this mixture with remaining cream, Nutella, and salt. Cook over medium heat, whisking constantly, until boiling. Boil for 3 minutes until thickened. Remove from heat and whisk in vanilla.

Assemble and Chill:
Pour filling into the cooled crust. Swirl the top if desired. Chill for 3–4 hours until set. Top with remaining hazelnuts before serving.