Preheat your oven to 350°F (175°C).
In a bowl, mix flour, cocoa powder, and powdered sugar.
Add cold butter and mix until crumbly.
Stir in the egg yolk and form a dough.
Press the dough into a tart pan evenly.
Bake the crust for 15 minutes, then cool slightly.
Heat the cream in a saucepan until just simmering.
Pour the hot cream over the chopped chocolate and let it sit for 5 minutes.
Stir until smooth, then whisk in the eggs and vanilla.
Pour the chocolate mixture into the cooled crust and bake for an additional 10 minutes.
Let it cool completely before serving.