Prepare the tart crust: In a mixing bowl, combine flour, cocoa powder, powdered sugar, and salt. Add the cubed butter and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
In a small bowl, whisk together the egg yolk and cold water. Add to the flour mixture and mix until dough forms. If too crumbly, add more water, one teaspoon at a time.
Press the dough into a 9-inch tart pan evenly across the bottom and up the sides. Prick the bottom with a fork. Chill in the refrigerator for 30 minutes.
Preheat the oven to 350°F (175°C). Bake the tart shell for 18-20 minutes or until set. Let it cool completely.
Make the salted caramel: In a saucepan over medium heat, melt the sugar until it turns to a golden amber color, stirring gently. Once melted, add the butter and whisk until combined. Slowly pour in the heavy cream while whisking continuously until smooth. Finally, stir in the sea salt. Remove from heat and let it cool slightly.
Pour the salted caramel into the cooled tart shell and spread it evenly. Refrigerate for at least 1 hour to set.
Prepare the chocolate ganache: In a saucepan, heat the cream until just simmering. Remove from heat and pour over the chopped chocolate in a bowl. Let it sit for 5 minutes, then stir until smooth. Stir in vanilla extract.
Pour the ganache over the set caramel layer in the tart. Use a spatula to spread evenly.
Refrigerate for an additional 2 hours or until the ganache is firm.
Before serving, sprinkle a little extra sea salt on top for garnish.
Notes
Sprinkle extra sea salt on top before serving for added flavor.