In a large bowl, whisk together flour, cocoa powder, and sugar. Cut in the butter until the mixture resembles coarse crumbs. Stir in the egg yolk and cold water until the dough comes together. Press the dough into a tart pan and refrigerate for 30 minutes.

Bake the Tart Crust

Preheat the oven to 350°F (175°C). Blind bake the tart crust for 15-20 minutes. Remove from oven and let cool.

Prepare the Chocolate Filling

In a small saucepan, heat heavy cream until just simmering. Pour over the chopped dark chocolate and let sit for 5 minutes. Stir until smooth. Stir in the chopped hazelnuts and pour the mixture into the cooled tart crust.

Make the Salted Caramel

In a saucepan, combine granulated sugar and water. Cook over medium heat without stirring until the mixture turns a deep amber color. Remove from heat and carefully stir in heavy cream and sea salt. Let cool slightly.

Assemble the Tart

Pour the salted caramel over the chocolate hazelnut filling. Spread evenly and let set in the refrigerator for at least 2 hours before serving.