In a large mixing bowl, combine the softened unsalted butter and powdered sugar. Beat until the mixture is light and fluffy. Add in the Dutch-process cocoa powder, all-purpose flour, egg yolks, and vanilla extract. Mix until a dough forms. If it feels too crumbly, add a teaspoon of cold water.
Press the dough evenly into the bottom and up the sides of your 9-inch tart pan. Use your fingers or the back of a measuring cup to smooth it out. Once shaped, cover the tart shell with plastic wrap and refrigerate for at least 30 minutes.
Preheat your oven to 350°F (175°C). Remove the tart shell from the refrigerator and prick the bottom with a fork to prevent air bubbles. Bake for 15-20 minutes until set and slightly firm. Allow it to cool completely before adding the filling.
In a saucepan over medium heat, combine the superfine sugar and water. Stir gently until the sugar dissolves. Allow it to cook without stirring until it turns a deep amber color, about 8-10 minutes. Remove from heat and carefully whisk in the 6 tablespoons of unsalted butter and 1 cup of heavy cream. Add the fleur de sel and stir until smooth.
Pour the caramel into the cooled tart shell, smoothing it out with a spatula. Let it cool to room temperature, then refrigerate for at least 2 hours to set.
In a heatproof bowl, place the finely chopped dark chocolate. In a small saucepan, heat the remaining 1/2 cup of heavy cream until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for a minute. Stir until the chocolate is completely melted and the ganache is smooth.
Once the caramel layer is set, pour the chocolate ganache over the caramel layer, spreading it evenly with a spatula. Sprinkle a pinch of fleur de sel on top for that perfect finishing touch.
Return the tart to the refrigerator for at least another hour before slicing. When ready to serve, carefully remove the tart from the pan and slice it into wedges. Enjoy the rich flavors of this Salted Chocolate Caramel Tart!