For the Chocolate Crust:
In a food processor, pulse together flour, cocoa powder, powdered sugar, and salt until combined.
Add cold cubed butter and pulse until mixture resembles coarse crumbs.
Add egg yolk and pulse. If needed, add ice water one tablespoon at a time until dough just comes together.
Form dough into a disk, wrap in plastic, and refrigerate for at least 1 hour.
Preheat oven to 350°F (175°C).
Roll chilled dough to 3mm thickness and press into a 9-inch tart pan with removable bottom.
Prick the bottom with a fork, line with parchment paper, and fill with pie weights or dried beans.
Blind bake for 15 minutes, then remove weights and parchment and bake for another 5-7 minutes until fully cooked.
Allow crust to cool completely before filling.
For the Salted Caramel:
In a heavy saucepan, combine sugar and water. Heat over medium heat without stirring.
Once sugar begins to color, swirl the pan gently until it reaches an amber color (about 350°F on a candy thermometer).
Remove from heat and carefully whisk in butter until melted and combined.
Slowly add warm heavy cream, whisking constantly (mixture will bubble vigorously).
Return to low heat and cook for 1 minute, stirring constantly.
Remove from heat and stir in sea salt. Allow to cool slightly to 200°F.
Pour caramel into cooled tart shell, creating an even layer. Refrigerate for 2-3 hours until set.
For the Chocolate Ganache:
Place chopped chocolate in a heatproof bowl.
Heat heavy cream in a saucepan until it just begins to simmer (do not boil).
Pour hot cream over chocolate and let stand for 2 minutes.
Gently stir from the center outward until smooth and glossy.
If using, add butter and stir until melted and incorporated.
Allow ganache to cool slightly until just pourable but not hot.
Pour ganache over the set caramel layer, spreading to the edges.
Refrigerate for at least 1 hour until ganache is set.
Before serving, sprinkle with additional flaky sea salt.