Make the Tart Crust: In a mixing bowl, combine the flour, cocoa powder, and powdered sugar. Add the cold cubed butter and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.
Add the egg yolk and ice water to the mixture, stirring until it forms a dough. If needed, add a little more ice water to bring the dough together.
Press the dough into a 9-inch tart pan evenly across the bottom and up the sides. Prick the bottom with a fork to prevent bubbling.
Chill the crust in the refrigerator for at least 30 minutes. Preheat your oven to 350°F (175°C).
Blind Bake the Crust: Line the chilled tart shell with parchment paper and fill with pie weights or dried beans. Bake in the preheated oven for 15 minutes, then remove the weights and parchment and bake for an additional 10-12 minutes until set. Let it cool completely.
Prepare the Salted Caramel: In a medium saucepan over medium heat, melt the granulated sugar, stirring constantly until it turns a deep amber color. Be careful not to burn it.
Once the sugar is melted, remove from heat and slowly whisk in the heavy cream (it will bubble up). Add the sea salt and stir until smooth. Let it cool slightly.
Make the Chocolate Ganache: In another saucepan, combine the chopped dark chocolate and butter. Heat gently until melted and smooth, stirring frequently.
Pour the caramel sauce into the cooled tart crust, then carefully pour the chocolate ganache on top. Use a spatula to spread it evenly.
Chill the tart in the refrigerator for at least 2 hours or until set.
Notes
Before serving, sprinkle flaky sea salt over the top of the tart for a beautiful contrast and garnish with chocolate shavings or a dollop of whipped cream.