Dark Chocolate Ganache Tart
In a small bowl, stir together the egg yolk, water and 2 tsp. of the vanilla; set aside.
In a stand mixer fitted with the flat beater, stir together the flour, sugar, cocoa powder and salt on low speed. Add the butter, increase the speed to medium-low and mix until the texture resembles coarse cornmeal, with butter pieces no larger than small peas. Add the egg mixture and beat just until the dough pulls together.
Transfer the dough to a work surface and flatten into a disk. Wrap in plastic wrap and refrigerate until well chilled, at least 1 hour.
Preheat an oven to 350°F (180°C).
On a lightly floured work surface, roll out the dough into a 20-by-8-inch (50-by-20-cm) rectangle. Transfer to a 13-by-4-inch (33-by-10-cm) tart pan with a removable bottom. Press the dough into the bottom and up the sides of the pan. Cut off any overhanging dough and prick the dough in several places with a fork.
Line the dough with parchment paper and fill with pie weights. Bake until the sides are cooked through and the bottom is almost cooked through and slightly flaky, 25 to 30 minutes. Remove the pie weights and parchment and continue to cook until the bottom is dry and set, about 8 minutes more. Transfer the pan to a wire rack and let cool.
Cut the remaining 2 Tbs. butter into small pieces and put in a heatproof bowl along with the chocolate. In a small saucepan over medium-high heat, bring the cream to a boil. As soon as it comes to a boil, remove the cream from the heat and pour it over the chocolate and butter. Let stand for 2 minutes, add the remaining 1 tsp. vanilla extract and whisk until smooth.
Carefully pour the chocolate ganache into the prepared tart shell. Sprinkle with fleur de sel to taste and refrigerate the tart until the filling is set, about 1 hour. Remove the tart from the refrigerator about 15 minutes before serving. Serves 8 to 10.