Make the Crust:

1. Prepare the dry ingredients:
   • In a large bowl, whisk together the all-purpose flour, hazelnut flour, sugar, and salt.
2. Cut in the butter:
   • Add the cold, cubed butter to the dry ingredients. Use a pastry cutter or your fingers to work the butter into the flour until the mixture resembles coarse crumbs.
3. Form the dough:
   • Add the egg yolk and 2 tablespoons of cold water. Mix until the dough comes together. If it’s too dry, add an additional tablespoon of water. Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
4. Bake the crust:
   • Preheat your oven to 375°F (190°C). Roll out the chilled dough on a lightly floured surface to fit a 9-inch tart pan. Press the dough into the pan and trim the edges.
   • Prick the bottom with a fork, line with parchment paper, and fill with pie weights or dried beans. Bake for 15 minutes, then remove the weights and bake for another 5–7 minutes, or until lightly golden. Let cool completely.

Make the Filling:
5. Melt the chocolate:

• In a heatproof bowl, add the chopped dark chocolate.

6. Heat the cream and milk:
   • In a small saucepan, heat the heavy cream and milk over medium heat until just simmering. Pour the hot cream mixture over the chocolate. Let sit for 2 minutes, then stir until smooth and glossy.
7. Finish the ganache:
   • Stir in the butter and vanilla extract until fully incorporated.

Assemble the Tart:
8. Fill the crust:

• Pour the chocolate ganache into the cooled tart crust, spreading it evenly.

9. Add toppings:
   • Sprinkle the top with chopped toasted hazelnuts and any additional toppings like shaved chocolate or caramel drizzle.
10. Chill and serve:
    • Refrigerate the tart for at least 2 hours, or until the ganache is set. Slice and serve chilled or at room temperature.