Add the flour, cocoa, icing sugar and butter to a food processor and work until the ingredients form a breadcrumb consistency (don't worry if some larger pieces of butter remain).
Add the egg yolk and work until a dough forms.
If you need a touch of cold water to help bind the ingredients, then add a little at a time (a tablespoon is a good start), just until you are left with a not too sticky or too dry dough (see note 3).
Form the dough into a ball and cover with clingfilm.
Place in the fridge for around 30-60 minutes.
Pre-heat the oven to 200°C (400℉).
On a floured surface, roll the dough out into a large circle, then transfer to a baking tin or pie dish (see note 4).
Trim the edges and place tin foil or baking paper in the bottom and fill with dried peas or whatever else you use to blind bake.
Place in the oven to bake for 15 minutes.
Remove the foil and dried goods and bake for a further five minutes.
Take out of the oven, and reduce the temperature to 175°C (350℉).
Allow the crust to cool for a while. You could leave this overnight if you preferred, or even freeze the pastry case for another time.