Step 1: For the crust, in a food processor, pulse toasted hazelnuts until finely ground. Add flour, sugar, and salt; pulse to combine.
Step 2: Add cold, cubed butter and pulse until mixture resembles coarse meal with pea-sized butter pieces.
Step 3: Gradually add ice water, 1 tablespoon at a time, pulsing until dough just comes together. Do not overmix.
Step 4: Form dough into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
Step 5: Roll out chilled dough to a 12-inch circle. Carefully transfer to a 9-inch tart pan with a removable bottom. Press dough into pan, trim edges, and prick bottom with a fork.
Step 6: Freeze tart shell for 15-20 minutes. Preheat oven to 375°F (190°C).
Step 7: Line frozen shell with parchment, fill with pie weights, and blind bake for 15 minutes. Remove weights/parchment and bake for another 8-10 minutes until lightly golden. Cool completely.
Step 8: For the filling, place finely chopped dark chocolate in a heatproof bowl.
Step 9: In a small saucepan, heat heavy cream and corn syrup (if using) over medium heat until it just simmers around the edges. Do not boil.
Step 10: Pour hot cream over chopped chocolate. Let sit for 5 minutes, then whisk gently from the center until smooth and glossy. Stir in hazelnut liqueur (if using) and a pinch of sea salt.
Step 11: Pour warm ganache into the cooled tart shell. Gently tap the tart to release air bubbles.
Step 12: Refrigerate for at least 3-4 hours, or until firm and set. Carefully remove from pan.
Step 13: Garnish with toasted hazelnut halves, cocoa powder, or flaky sea salt before serving.