To make the crust: In a food processor, place ½ cup hazelnuts (reserving the remaining ¼ cup for garnishing), flour, sugar, and salt. Pulse for a few seconds until nuts are finely ground. Add butter and pulse until mixture becomes crumbly and resembles coarse meal, about 15 pulses. Add egg and vanilla extract and keep pulsing until the dough is no longer dry and starts to clump together, about 10-15 seconds. The dough should be crumbly with large clumps. Turn dough to a lightly floured surface and form into a ball. Flatten ball slightly with your hands to form a thick disc. Wrap with plastic wrap and refrigerate for 1 hour.

Preheat oven to 350°F/175°C and place rack in the center.

To roll out the dough: On a lightly floured surface (or for easier rolling, place dough between two parchment papers - watch video), roll the dough out into a 12-inch circle, then place gently into a 9-inch tart pan with a removable bottom. You can do this by flouring a rolling pan and rolling the dough loosely around it, then unrolling it into the pan. With a sharp knife, trim the edges of the pastry to fit the tart pan. Press parchment paper or aluminum foil tightly against the crust, covering the edges to prevent them from burning. Fill with pie weights(or dried beans/uncooked rice), making sure they’re fully distributed over the entire surface. Bake crust for 20 minutes, then gently remove weights and foil, then bake for about 10 minutes longer until golden brown. Transfer to a wire rack and let cool completely.

Meanwhile, make the filling: Whisk the cornstarch and ¼ cup heavy cream in a bowl until smooth. Combine the remaining 1 and ¾ cups cream, Nutella, vanilla and salt in a small saucepan. Whisk in the cornstarch mixture and bring to a boil over medium heat. Once the mixture starts to boil, stir constantly until thickened, about 2 minutes. Pour into the crust. Refrigerate until set, at least 3 hours. Top with the reserved hazelnuts.

Tart will keep, covered well, for up to 5 days in the fridge.