Place the flour in the bowl of a stand mixer fitted with the paddle attachment. Add the butter gradually and mix on low speed until the mixture resembles coarse crumbs, like couscous. Add the powdered sugar, egg yolk, salt, and vanilla. Mix just until a dough begins to form. If the mixture looks dry or crumbly, add 1 teaspoon of cold water and mix briefly until cohesive.
Shape the dough into a flat disk, wrap tightly in plastic wrap, and refrigerate for 1 hour.
Roll the chilled dough between two sheets of parchment paper into a round about 11 inches/28 cm in diameter. Line a 9-inch/23 cm tart pan with a removable bottom, gently pressing the dough into the corners and up the sides. Trim any overhang. Prick the base all over with a fork. Freeze the tart shell for 30 minutes.
Preheat the oven to 340°F (170°C). Line the frozen shell with parchment paper and fill with pie weights or dried beans. Bake for 10 minutes. Carefully remove the parchment and weights, then return the tart shell to the oven and bake for another 10 to 12 minutes, or until golden. Let cool completely on a wire rack.
Place the chopped chocolate in a heatproof bowl. In a small saucepan, bring the cream to a gentle boil over medium heat. Immediately pour the hot cream over the chocolate and let stand for 30 seconds. Stir slowly with a spatula until smooth, glossy, and fully emulsified.
Pour the ganache into the cooled tart shell. Tilt the pan slightly to distribute the filling evenly. Refrigerate for at least 3 hours, or until fully set. Slice with a hot, dry knife and serve chilled.
Store the tart covered in the refrigerator for up to 4 days.