1. Make the crust: In a food processor, combine flour and powdered sugar. Add cold butter cubes and pulse until mixture resembles coarse crumbs. Add egg yolk and cold water; pulse until dough starts to come together.

2. Chill the dough: Form dough into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

3. Preheat oven: Set oven to 350°F (175°C).

4. Bake the crust: Roll out dough on a floured surface and press into a 9-inch tart pan. Prick the bottom with a fork, line with parchment paper, and fill with pie weights or dried beans. Bake for 15 minutes, remove weights and paper, then bake another 10 minutes until golden. Let cool.

5. Make salted caramel: In a saucepan over medium heat, melt granulated sugar, stirring constantly until it turns amber. Remove from heat, whisk in butter until smooth. Slowly add warmed heavy cream, stirring constantly. Stir in sea salt. Let cool slightly.

6. Make chocolate ganache: Heat heavy cream in a small saucepan until just simmering. Pour over chopped chocolate and let sit 2 minutes, then stir until smooth. Stir in vanilla extract.

7. Assemble tart: Pour chocolate ganache into cooled crust, smooth the top. Carefully pour salted caramel over the ganache layer.

8. Chill: Refrigerate tart for at least 2 hours or until set.

9. Serve: Garnish with sea salt flakes if desired. Slice and enjoy!