Preheat your oven to 350°F (175°C).
In a bowl, combine flour, cocoa powder, and powdered sugar.
Add the chilled butter and mix until crumbly.
Stir in the egg yolk and cold water until the dough forms.
Roll out the dough and press it into a tart pan.
Prick the base with a fork and chill for 15 minutes.
Bake for 15 minutes, then allow to cool.
Melt the dark chocolate in a double boiler.
In another bowl, whisk together cream, eggs, and vanilla.
Combine melted chocolate with cream mixture and pour into the crust.
Bake for an additional 10 minutes, then let cool.
Chill in the refrigerator for at least 2 hours before serving.