1. Cut butter into 1/2-inch cubes and place back in refrigerator.
2. To skin hazelnuts, preheat oven to 400°. Spread 1 1/2 cups nuts on a sheet tray and roast for 10 to 15 minutes, until the skin looks blistered. Wrap them in a dish towel to steam for 1 minute. Skins will easily pop off. Roughly chop hazelnuts and set aside 1/2 cup for topping tart.
3. Prepare Crust: Finely chop 1 cup hazelnuts in the bowl of a food processor.
4. Add flour, sugar, and salt to food processor and pulse until nuts are finely ground. Add cold butter, cream and yolks and pulse until dough just comes together.
5. Press over bottom and up sides of 10- or 11-inch tart pan with removable bottom. Pierce crust all over with fork. Chill at least 2 hours or overnight.
6. Bake Crust: Preheat oven to 400°. Crumple a 14-inch square piece of parchment paper into a ball. Line the chilled tart crust with the parchment and fill completely with pie weights or dried beans. Work quickly so tart crust stays cold. Bake crust for 20 minutes, until firm. Let cool completely.
7. Make Filling: Lower oven temperature to 325°. Heat cream and milk in a small saucepan to almost boiling. Do not boil.
8. Chop the chocolate into 1/2-inch pieces and place in a large bowl. Pour the hot cream mixture over chocolate and whisk until chocolate melts completely. Let cool for 10 minutes.
9. Stir eggs into chocolate until combined. Add Nutella and whisk until smooth. Pour chocolate filling onto crust.
10. Bake for 30 minutes until just set. Check once and cover crust with foil if crust turns golden brown. Let cool completely.
Decorate tart with chopped hazelnuts. Garnish with Maldon sea salt, shaved chocolate, Heath bars, or whipped cream as desired. Enjoy!