Sift the flour, cocoa powder, and icing sugar into a large bowl.
Add the cold butter and work it into the flour mixture by rubbing your palms together through the mixture until the mixture resembles fine breadcrumbs.
Make a well in the center and add the yolks. Work the yolks into the dough with a fork or even your fingers.
If the dough is slightly dry, add a spoonful or so of cold water until the desired texture is obtained.
Form the dough into a disk and wrap it in plastic wrap. Refrigerate for at least 1 hour or until firm.
Preheat the oven to 350°F.
Roll out the disk of dough between two pieces of parchment until it is 3 mm thick.
Line a 9-inch removable bottomed tart pan with the dough and work it into the corners and edges. Trim and neaten it up.
Freeze the unbaked tart for half an hour or until very, very cold.
Dock the pastry with a fork and with a 6 mm piping tip, cut out a whole in the center of the tart to allow steam to escape (make sure to cut all the way through and remove the piece).
Place the tart on a baking sheet and blind bake the tart for about 15–20 minutes until the pastry is fully cooked and appears dry (not glossy).
Remove the tart from the oven. If there are any bumps of air under the surface of the tart shell, press down very gently with a clothed hand (beware of steam) to release the steam. Let cool before unmolding it carefully and placing it on a plate.