Step 1In a small bowl, lightly beat the egg yolk with the vanilla. Add 1 tablespoon orange zest and set aside.
Step 2Place the flour, sugar and salt in the bowl of a food processor and pulse briefly to blend the ingredients. Add the butter and pulse for about 30 seconds, or until it is cut into tiny pieces. With the processor running, add the yolk mixture through the feed tube. Mix for about 1 minute, or until the dough forms a ball and wraps around the blade.
Step 3Transfer the dough to a board and shape into a 6-inch disk. Wrap in a double layer of plastic wrap and refrigerate until firm, about 3 hours. (The dough can be refrigerated for up to 4 days or frozen for up to 2 months. If frozen, defrost the dough overnight in the refrigerator.)
Step 4Remove the dough from the refrigerator and let it temper for just a few minutes if the dough is too hard to roll. Roll the dough between two sheets of lightly floured parchment paper into a 13-inch circle. Flour the dough as necessary to keep it from sticking. Remove the top sheet of parchment paper and brush off any excess flour. Roll the dough up around the rolling pin and unroll it into the tart pan. Gently lift the edges to ease the dough into the corners of the pan, then push down gently. Trim the dough to a 1-inch overhang. Fold the overhang over and press it against the sides of the pan to form a secure double layer. If the dough tears, it is easy to press and patch into place. Place the tart pan on a baking sheet and refrigerate for 20 minutes.
Step 5Position a rack in the center of the oven and heat it to 375 degrees. Prick the bottom of the tart shell with a fork. Line the shell with aluminum foil or parchment and fill with pie weights or dry beans. Bake for 12 minutes.
Step 6Remove the parchment and weights, prick the bottom of the crust again, and bake for 15 to 20 minutes longer, or until golden. Remove the pan from the oven and cool completely on a rack.
Step 7To prepare the hazelnuts for garnish, reduce the oven temperature to 350 degrees, place the nuts in a baking dish and toast until lightly browned, about 10 minutes.
Step 8For the ganache, place the chocolate in a medium bowl and set a fine-mesh strainer over the bowl. In a small saucepan, bring the cream to a boil over medium-high heat. Add the remaining 1 teaspoon zest to the cream, turn off the heat and cover the pan. Let steep for 15 minutes. Remove the lid and bring the cream back to a boil over medium-high heat. Strain onto the chocolate. Let stand for 30 seconds, then use a whisk, rubber spatula, or immersion blender to blend the mixture until very smooth.
Step 9Pour the ganache into the tart shell and spread it evenly. Sprinkle the top with the hazelnuts or zest. Let stand at room temperature for 3 to 4 hours, or refrigerate for about 1 hour, until the ganache is set. (The tart can be made ahead and stored at room temperature for up to 2 days. Once the ganache is set, cover the top with a piece of parchment paper and wrap the entire tart with aluminum foil.) To serve, remove the ring of the tart pan, sprinkle with toasted hazelnuts and cut the tart into thin slices.