In a large mixing bowl, combine the all-purpose flour, cocoa powder, and powdered sugar using a whisk until well blended. Add the chilled and cubed butter into the dry mixture. Use your fingers to rub the butter into the flour mixture until it has a coarse crumb texture. Incorporate the egg yolk into the mixture. Gradually add cold water, one tablespoon at a time, mixing until a dough begins to form. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for 30 minutes.
Preheat your oven to 350°F (175°C). On a lightly floured surface, roll out the chilled dough to fit a 9-inch tart pan. Carefully press the rolled dough into the tart pan, trimming any excess off the edges. Using a fork, prick the bottom of the tart shell to prevent bubbling. Line the base with parchment paper and fill with pie weights or dried beans. Bake in the preheated oven for 15 minutes. Remove the parchment paper and weights, and bake for an additional 10 minutes until firm. Allow the tart shell to cool completely on a wire rack.
In a medium saucepan over medium heat, add the granulated sugar. Stir continuously until the sugar melts and develops a rich golden amber hue. Once melted, carefully add the cubed butter (watch out for splattering), stirring until fully melted and combined. Slowly incorporate the heavy cream into the mixture, stirring until it becomes smooth and glossy. Take off the heat and mix in the sea salt.
Pour the warm salted caramel into the cooled tart shell, spreading it evenly to create a smooth layer. Place the tart in the refrigerator for at least 30 minutes to let the caramel set properly.
In a small saucepan, heat the heavy cream over low heat until it starts to simmer. Remove from heat and pour over the dark chocolate chips in a separate bowl. Let it sit untouched for about one minute, then stir gently until the chocolate is fully melted and smooth. Add the vanilla extract and stir to combine.
Gently pour the chocolate ganache over the chilled caramel layer within the tart shell. Use a spatula to smooth the top evenly. Allow the tart to sit at room temperature briefly, then refrigerate for an additional 1-2 hours until the ganache is set.
Before serving, lightly sprinkle flaked sea salt over the top for a gorgeous contrast. If desired, dust a little cocoa powder for an extra touch of elegance. Slice the tart into wedges, serve chilled, and relish the decadent layers of flavor!