To make the crust combine the flour, cocoa powder, sugar, and salt in a bowl.  Mix to combine.

Add in the butter and use your fingers to mix it into the dry ingredients until the mixture looks like sand.

Add the egg yolk and one tablespoon of cold water and mix until the dough comes together and you can form a ball.  I used my hands to do this.  Add a teaspoon more of water if needed.

Press the dough into a 9 inch tart pan in an even layer.  Refrigerate for at least 30 minutes or overnight.

Preheat the oven to 350 degrees.  Line the tart crust with parchment paper and fill with dried beans or pie weights.

Bake for 13-15 minutes or until the crust looks dry.  Remove the
parchment paper and weights and then bake for an additional 5 minutes.

Transfer to a wire rack and cool completely.

To make the filling combine the sugar and water in a medium saucepan over medium high heat. Cook until the sugar is disolved, about 5 minutes.

Watch the pop carefully, not stirring, until the caramel is a dark amber color.

Remove the caramel from the heat and add in the heavy cream. Whisk constantly until the caramel stops bubbling. Stir in the vanilla and sea salt. Cool to room temperature.

Meanwhile, combine the chocolate and butter in a heatproof bowl over top of a saucepan with an inch of simmering water. Let the chocolate and butter melt for 5 minutes then begin to sstir. Mix until combined and totally melted.

Remove the bowl from the top of the saucepan and let cool.

Preheat the oven to 325 degrees.

In a large bowl whisk the egg yolks and the whole egg for 1-2 minutes or until frothy. Slowly whisk in the cooled caramel and the melted chocolate.

Set the tart pan on a rimmed baking sheet and then pour the filling into the tart.

Bake for 25-30 minutes or until the tart is set around the edges but the center is still a bit jiggly. Remove from oven and transfer to a wire rack to cool for 30 minutes.

Refrigerate for 3 hours or until completely chilled.

Sprinkle with flaky sea salt and powdered sugar before serving.