Add the flour, cocoa, icing sugar and butter to a food processor and work until the ingredients form a breadcrumb consistency.
Add the egg yolk and work until a dough forms. If you need a touch of cold water, add a little at a time until you are left with a not too sticky or too dry dough (I didn't need any water at all - at first it might seem that one egg yolk is not enough liquid but it probably is, so make completely sure before you add any water).
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.
On a floured surface, roll the dough out into a large circle, then transfer to a baking tin or pie dish. I used a very shallow pie dish with a removable bottom that is 27cm in diameter. I had pastry to spare, so you could easily make a larger pie, but you would need to adapt the filling measurements.
Trim the edges and place tin foil or baking paper in the bottom and cover with dried peas or whatever else you use. Blind bake for 15 minutes. Remove the foil and dried goods and bake for a further five minutes. Remove from the oven (reduce the temperature to 175°C) and allow to cool for a while (you could leave this overnight, if you preferred or even freeze the pastry case for another time).