Put the flour, sugar and salt in a bowl and add butter. Rub into your hands until you get a coarse bread crumb consistency, making sure there aren't any large lumps of butter left.
Add the eggyolk, vanilla and water and mix until the dough comes together, being careful not to mix any longer than necessary. You might need to add a tiny amount of additional water.
Remove the dough from the mixing bowl and knead very lightly for a few seconds only, just to shape into a smooth disk, about 5 to 6 cm thick. Wrap in plastic and chill until ready to use.
Continue by brushing the tartlet pans with a thin layer of melted butter, then leave set in the fridge.
Meanwhile, lightly dust the work surface with flour and roll out the pastry thinly. Once the pastry in no more than 2 to 3 mm thick, cut out 6 circles using a pastry cutter. Place the pastry circles inside the buttered tartlet pans, pressing them into the sides. Let rest in a fridge for at least 30 minutes.
Preheat the oven to 300 F. Top each dough circle with a paper muffin liner, and fill it with a thin layer of dried beans or rice,
Bake for about 20 minutes until just golden, and remove from the oven when finished. Let cool for five minutes.
Discard the rice-filled muffin liners. Remove the tart shells from the pan while still warm, let cool on a cooling rack.
Dark Chocolate Filling
Preheat the oven to 325 F. Put the chocolate and butter in a bowl, set it over a pan of simmering water, and leave to melt.
In a medium bowl, whisk the egg and yolk with sugar until thick and pale, then fold the mixture into the melted chocolate.
Fill the tartlet up with the chocolate mix; it should reach right up to the rim.
Place in the oven and bake for 5 to 7 minutes at 325 F. Cool a little, then remove the tartlets from their pans and allow them to cool completely.
Lightly dust with cocoa powder and fleur de sel, and serve at room temperature.