Make the Tart Shell: In a large mixing bowl, whisk together the flour, cocoa powder, and powdered sugar. Add the cubed butter and rub it with your fingers until the mixture resembles coarse crumbs.
Add the egg yolk and gradually mix in cold water, 1 tablespoon at a time, until the dough comes together. Shape it into a disk, wrap in plastic, and refrigerate for 30 minutes.
Preheat your oven to 350°F (175°C). On a floured surface, roll out the dough to fit a 9-inch tart pan. Press it into the pan and trim the edges. Prick the bottom with a fork and line it with parchment paper. Bake for 15 minutes, then remove the paper and bake for another 10 minutes. Allow to cool completely.
Prepare Salted Caramel: In a medium saucepan over medium heat, melt the granulated sugar, stirring constantly until it turns a golden amber color. Carefully add the cubed butter (it may bubble) and stir until melted. Slowly pour in the heavy cream while continuing to stir until smooth. Remove from heat and stir in the sea salt.
Pour the caramel into the cooled tart shell, spreading it evenly, and place in the refrigerator to set for at least 30 minutes.
Make Chocolate Ganache: In a small saucepan, heat the heavy cream until it just begins to simmer. Remove from heat and pour over the chocolate chips in a bowl. Let it sit for a minute, then stir until smooth. Add the vanilla extract.
Pour the chocolate ganache over the set salted caramel layer in the tart shell, smoothing it out with a spatula. Allow it to cool at room temperature, then refrigerate for another 1-2 hours to fully set.
Serve: Before serving, sprinkle flaked sea salt on top for garnish. Optionally, dust with cocoa powder for an elegant finish. Slice and enjoy!
Notes
Allow the tart to chill completely for the best texture.