Add the flour, cocoa powder and powdered sugar to the bowl of a food processor and blitz for 20 seconds to combine. Add the butter and pulse until you reach lentil sized pieces. Add the egg yolks and blitz until small clumps of dough form. Keep the egg whites because we’ll be using them for the meringue!
Turn the crumbly mixture out your workbench and bunch it together (without kneading). Form a rough disc shape and then wrap in plastic. Chill for 60 minutes.
Preheat your oven to 180°C / 355°F.
Dust your workbench with flour and use a rolling pin to roll out to 14 inches wide. Carefully roll over the rolling pin and then transfer and unroll over a 9-inch tart tin with removable bottom. Carefully lift the dough over the tart to allow it to naturally fall into the corners of the tart. You don’t want to stretch it or it will shrink as it bakes and become uneven. Discard excess dough.
Place the pie tin on a baking tray. Scrunch up a large piece of baking paper and then un scrunch it. Place on the inside of the tart tin. Fill with baking beads. I used dried chickpeas. Bake for 15 minutes. Then take it out of the oven, carefully lift out the beads using the baking paper and bake on 150°C / 300°F for a further 10 minutes. Separate one of the eggs.Brush the hot tart shell with a tin layer of egg white and set aside to cool. This will help keep the tart shell crispy.