Make the crust: Toast the hazelnuts in a skillet over medium heat until golden, about 8 minutes. Let cool. Transfer 1/3 cup nuts to a food processor. Add the flour, sugar and salt; pulse until the nuts are finely ground. Add the butter and pulse until the mixture looks like coarse meal. Drizzle in the egg and vanilla; pulse until the dough starts to come together.
Turn the dough out onto a sheet of plastic wrap and pat into a disk. Wrap and refrigerate until firm, at least 1 hour. Coarsely chop the remaining hazelnuts and reserve.
Roll out the dough on a lightly floured surface into a 12-inch round. Press into the bottom and sides of a 9-inch tart pan with a removable bottom, then trim the excess dough. Prick the bottom all over with a fork. Refrigerate until firm, about 30 minutes.
Preheat the oven to 350 degrees F. Line the crust with foil, then fill with pie weights or dried beans. Bake until the edges are golden, about 20 minutes. Remove the foil and weights and continue baking until golden brown all over, 15 to 20 minutes. Cool completely on a rack.
Meanwhile, make the filling: Whisk the cornstarch into 1/4 cup cream in a bowl. Combine the remaining 1 3/4 cups cream, the chocolate-hazelnut spread, vanilla and salt in a small saucepan. Whisk in the cornstarch mixture and bring to a boil over medium heat, stirring and scraping the sides of the pan with a rubber spatula. Once the mixture starts to boil, stir constantly until thickened, about 2 minutes. Pour into the crust and swirl the top. Refrigerate until set, about 1 hour. Top with the reserved hazelnuts.