In a large mixing bowl, combine the flour, cocoa powder, granulated sugar, and salt. Cut in the unsalted butter until the mixture resembles coarse crumbs. Add the egg yolk and mix until the dough forms. Press into a 9-inch tart pan and chill.

Bake the crust in a preheated oven at 350°F (175°C) for about 20 minutes. Allow to cool completely.

Chop the semisweet chocolate and place it in a heatproof bowl. Heat heavy cream and butter until simmering, pour over chocolate, and whisk until smooth. Stir in vanilla extract.

Pour the chocolate filling into the cooled tart crust, spread evenly, and let set at room temperature for 30 minutes. Refrigerate for at least 2 hours.

Combine granulated sugar and water in a saucepan, cook until amber. Remove from heat and whisk in heavy cream and sea salt until smooth.

Pour salted caramel over the chocolate filling, refrigerate for an additional 30 minutes to firm up.

Slice the tart into wedges, sprinkle extra sea salt on top before serving.