1. Prepare the Hazelnut Crust: In a food processor, pulse together flour, hazelnut flour, sugar, and salt until combined. Add cold butter cubes and pulse until mixture resembles coarse crumbs. Add egg yolk, 2 tablespoons ice water, and vanilla extract. Pulse just until dough begins to come together (add the additional tablespoon of water only if needed). Turn dough onto a lightly floured surface, form into a disc, wrap in plastic, and refrigerate for at least 1 hour.
2. Blind Bake the Crust: Preheat oven to 375°F (190°C). Roll out the chilled dough on a lightly floured surface to about ⅛-inch thickness. Transfer to a 9-inch tart pan with a removable bottom, pressing gently into the corners. Trim excess dough. Prick the bottom all over with a fork. Line with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, then remove weights and parchment. Return to oven and bake for an additional 5-7 minutes until light golden. Reduce oven temperature to 350°F (175°C).
3. Make the Chocolate Filling: Place chopped chocolate in a medium heat-proof bowl. In a saucepan, bring heavy cream just to a simmer over medium heat. Pour hot cream over chocolate and let stand for 1 minute, then whisk until smooth. Add butter and whisk until melted and incorporated. Stir in hazelnut liqueur or hazelnut spread and salt. Let cool for 5 minutes. In a small bowl, lightly beat the eggs, then gradually whisk into the chocolate mixture until well combined.
4. Bake the Tart: Pour the chocolate filling into the pre-baked crust. Bake at 350°F (175°C) for 20-25 minutes, until the filling is set around the edges but still slightly jiggly in the center (it will continue to set as it cools). Remove from oven and let cool on a wire rack for at least 1 hour.
5. Add the Toppings: Once the tart has cooled completely, drizzle the warmed chocolate hazelnut spread over the top in a decorative pattern. Arrange the toasted chopped hazelnuts around the edge or over the entire surface. If desired, sprinkle with a small amount of flaky sea salt and dust lightly with cocoa powder.
6. Chill and Serve: Refrigerate the tart for at least 2 hours before serving to allow the filling to fully set. Remove from refrigerator about 20 minutes before serving to take the chill off for the best flavor and texture.

Serving Suggestions:

Serve slices of this decadent tart with a dollop of lightly sweetened whipped cream, a scoop of vanilla or hazelnut gelato, or fresh berries. For an elegant presentation, dust plates with cocoa powder before plating slices, and garnish with a whole toasted hazelnut and a small edible gold leaf. This tart pairs beautifully with coffee, espresso, or a glass of dessert wine.