Step 1: For the Tart Crust: In a food processor, combine the all-purpose flour, powdered sugar, and salt. Add the cold unsalted butter pieces and pulse until the mixture resembles coarse crumbs.
Step 2: Add the egg yolk and 2-3 tablespoons of ice water, one tablespoon at a time, pulsing until the dough just comes together. Be careful not to overmix.
Step 3: Gather the dough, flatten it into a disk, wrap it tightly in plastic, and chill in the refrigerator for at least 30 minutes.
Step 4: On a lightly floured surface, roll out the chilled dough to an 11-inch round. Carefully transfer it to a 9-inch tart pan with a removable bottom. Gently press the dough into the pan, trimming any excess.
Step 5: Prick the bottom of the crust all over with a fork. Freeze the tart shell for 15 minutes.
Step 6: Preheat your oven to 375°F (190°C). Line the frozen crust with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes. Remove the weights and parchment, then bake for another 5-7 minutes, or until the crust is lightly golden. Let cool completely.
Step 7: For the Hazelnut Paste and Chocolate Filling: Reduce oven temperature to 350°F (175°C). Spread the whole blanched hazelnuts on a baking sheet. Roast for 10-12 minutes, or until fragrant and lightly golden. If using unblanched hazelnuts, rub them vigorously in a clean kitchen towel immediately after roasting to remove most of the skins. Let cool.
Step 8: In a food processor, combine the roasted hazelnuts with the granulated sugar. Process until a smooth, oily paste forms, scraping down the sides as needed. This may take 5-7 minutes.
Step 9: Place the finely chopped dark chocolate in a heatproof bowl.
Step 10: In a medium saucepan, gently heat the heavy cream and milk until just simmering (do not boil). Pour the hot cream mixture over the chopped chocolate. Let stand for 5 minutes, then whisk until smooth and fully melted.
Step 11: In a separate small bowl, lightly whisk the 2 large eggs and vanilla extract. Gradually whisk the egg mixture into the chocolate mixture until well combined. Then, whisk in the homemade hazelnut paste until fully incorporated and smooth.
Step 12: Pour the chocolate-hazelnut filling into the cooled, blind-baked tart crust.
Step 13: Carefully transfer the tart to the preheated oven. Bake for 25-30 minutes, or until the edges are set but the center still has a slight wobble.
Step 14: Let the tart cool completely on a wire rack at room temperature. Once cooled, chill in the refrigerator for at least 2 hours (preferably overnight) before serving. Garnish with a dusting of cocoa powder or chopped hazelnuts if desired.