Place the flour and sugar in a food processor and pulse several times to combine the dry ingredients. Add the butter and pulse several more times until the mixture resembles coarse cornmeal.
In a liquid measuring cup, briefly whisk together the egg yolk, water and vanilla extract until frothy. With the machine running, pour the liquid in until the dough has just come together. Wrap the dough in plastic, giving it a few quick kneads first if the ingredients don’t look completely incorporated, and flatten into a disc. Chill for at least 2 hours.
On a lightly floured surface, roll the dough into a thin circle that is approximately 12 inches in diameter. Roll the dough around the rolling pin and then gently unroll it into a 9-inch tart pan, leaving slack so the dough can settle into the flutes. Gently shape the dough to the side of the pan and trim away any excess dough (see photos and images for a more thorough tutorial). Chill the dough for at least 15 minutes.
Preheat the oven to 375 degrees F. Use a fork to prick numerous holes in the dough to prevent air bubbles. Add a layer of aluminum foil and dried beans to weigh down the dough. Place the tart pan on a baking sheet and bake for approximately 18 - 20 minutes, removing the beans and foil halfway through, until the dough has just firmed up on the bottom. Allow to cool.
Turn the oven down to 350 degrees F. Spread the coconut in a thin layer on a baking sheet and toast until golden and fragrant, stirring once, 3-5 minutes. Set aside.
Place the bittersweet chocolate in a large bowl.
In a medium-sized saucepan, bring the cream to a gentle simmer over medium heat. Once it’s just simmering, pour over the chocolate and stir until very smooth. Whisk together the eggs, vanilla and sea salt and add to the chocolate, stirring until smooth. Pour into the pre-baked tart shell and bake for 25-28 minutes. The center of the tart should wobble just slightly while the sides should appear solid. It will continue to set as it cools. Remove from the oven.
While the tart is still warm, sprinkle the toasted coconut and coarse seat salt on top, very gently pressing into the chocolate to help adhere. Let the tart cool for 1 hour before serving, or cover and refrigerate for up to 12 hours.
Notes
This tart dough freezes very well. The progress photos show double the amount of tart dough required for the recipe. If you are new to working with dough, I highly recommend doubling the recipe so there is more to work with while rolling and shaping the tart. You can then wrap and freeze the remaining dough for up to 3 months.