Prepare the Crust

1. Make the Dough: In a food processor, combine the flour, cocoa powder, and powdered sugar. Add the cold butter and pulse until the mixture resembles coarse crumbs. Add the egg yolk and pulse again. Gradually add ice water, one tablespoon at a time, until the dough starts to come together.
2. Chill the Dough: Turn the dough out onto a floured surface and gently knead it into a ball. Flatten it into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
3. Roll the Dough: Once chilled, roll out the dough on a floured surface to about 1/8-inch thickness. Carefully fit the dough into a 9-inch tart pan, trimming any excess.
4. Blind Bake the Crust: Preheat the oven to 350°F (175°C). Line the crust with parchment paper and fill it with pie weights or dried beans. Bake for 15 minutes, then remove the paper and weights and bake for an additional 10 minutes, until the crust is firm and slightly browned. Let it cool completely.

2. Make the Chocolate Ganache

1. Heat the Cream: In a small saucepan, bring the heavy cream to a simmer over medium heat. Remove from the heat just before it starts to boil.
2. Prepare the Ganache: Place the chopped chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit for 3-4 minutes to soften. Stir gently until the chocolate is fully melted and the mixture is smooth. Add the butter and stir until incorporated. Let the ganache cool slightly.

3. Make the Caramel

1. Prepare the Caramel: In a medium saucepan, heat the granulated sugar over medium heat. Stir continuously until the sugar melts and turns a golden amber color.
2. Add Butter and Cream: Once the sugar has melted, carefully add the butter and stir until it’s fully melted. Slowly pour in the heavy cream while continuing to stir. Be careful as the mixture may bubble up. Stir in the vanilla extract and salt.
3. Let it Cool: Remove from heat and let the caramel cool for a few minutes.

4. Assemble the Tart

1. Fill the Tart Shell: Once the crust has cooled, pour the ganache into the tart shell, spreading it evenly.
2. Drizzle the Caramel: Pour the cooled caramel over the ganache, drizzling it gently for an elegant swirl.
3. Add the Hazelnuts: Sprinkle the toasted and chopped hazelnuts on top of the caramel layer. This adds a delightful crunch and nutty flavor that complements the sweetness of the caramel and the richness of the ganache.
4. Chill: Refrigerate the tart for at least 1 hour to allow the ganache and caramel to set.