In a large mixing bowl, combine the flour, cocoa powder, powdered sugar, and salt. Add the cold cubed butter and mix until the mixture resembles coarse crumbs. Add the egg yolk and ice water, mixing until the dough comes together. Press the dough into a tart pan and refrigerate for 30 minutes.

Bake the crust

Preheat your oven to 350°F (175°C). Remove the crust from the fridge and line it with parchment paper. Fill with pie weights or dried beans and bake for 15 minutes. Remove weights and parchment and bake for an additional 10 minutes until set. Let cool completely.

Caramel

Make the caramel

In a saucepan over medium heat, melt the granulated sugar until it turns a deep amber color. Carefully add the heavy cream and butter (it will bubble), stirring until smooth. Add the fleur de sel. Remove from heat and let cool slightly before pouring it into the cooled tart shell.

Ganache

Prepare the ganache

In another saucepan, heat the heavy cream until it just begins to simmer. Pour it over the chopped dark chocolate in a bowl and let it sit for 5 minutes. Stir until smooth. Pour the ganache over the caramel layer in the tart shell and spread evenly.

Chill and serve

Refrigerate the tart for at least 2 hours until set. Once set, slice and serve with a sprinkle of fleur de sel on top.