Prepare the Tart Crust: In a large mixing bowl, combine the all-purpose flour, cocoa powder, and powdered sugar, mixing well. Add the cold, cubed butter to the bowl. Using a pastry cutter or your fingers, blend the butter into the dry ingredients until the mixture resembles coarse crumbs, with small pea-sized pieces of butter remaining.
Form the Dough: Add the egg yolk and 1 tablespoon of ice water to the mixture. Stir gently until the dough begins to clump together. If the dough feels too dry, incorporate additional ice water, one teaspoon at a time, until it holds together.
Shape the Crust: Press the dough evenly into a 9-inch tart pan, making sure to cover the bottom and up the sides without any gaps. Use a fork to prick the bottom of the crust, which helps prevent bubbling during baking.
Chill the Crust: Place the tart shell in the refrigerator to chill for at least 30 minutes. During this time, preheat your oven to 350°F (175°C).
Blind Bake the Crust: Once chilled, line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake in the preheated oven for 15 minutes. After that, carefully remove the weights and parchment, then continue baking for an additional 10-12 minutes until the crust is set and slightly firm. Allow it to cool completely.
Prepare the Salted Caramel: In a medium saucepan, heat the granulated sugar over medium heat. Stir continuously until it melts and turns a deep amber color. Watch closely to prevent burning, adjusting the heat if necessary.
Incorporate the Cream: Once the sugar is fully melted and golden, remove the saucepan from the heat. Slowly whisk in the heavy cream, being cautious as it will bubble up. Stir in the sea salt until the mixture is smooth and well combined. Allow it to cool slightly at room temperature.
Make the Chocolate Ganache: In a separate small saucepan, combine the chopped dark chocolate and 4 tablespoons of butter. Heat over low, stirring frequently, until the chocolate has fully melted and is smooth, being careful not to overheat.
Fill the Tart: Pour the salted caramel sauce into the cooled tart crust in an even layer. Next, gently pour the chocolate ganache over the caramel, using a spatula to spread it evenly across the top.
Chill the Tart: Place the tart in the refrigerator to chill for at least 2 hours, or until set and the ganache is firm.
Notes
Before serving, sprinkle flaky sea salt over the top and consider adding chocolate shavings or whipped cream for decoration.