Sift the cocoa powder, flour, icing sugar and a pinch of salt into a food processor. Add the butter and process until it resembles fine crumbs. In a bowl, mix the egg yolk with 60ml of water and gradually pulse it into the flour mixture until you have a rough dough.
Place the dough on a clean surface and knead it for 30 seconds or until smooth, then flatten it into a disc. Wrap it in cling film and chill in the fridge for 30 minutes. After that, roll out the dough and use it to line a 23cm pie tin. Trim the excess pastry so you’re left with 2.5cm overhang. Tuck this under, ensuring it’s level with the rim to give a raised edge, and crimp with your fingers and thumb. Prick the base with a fork and pop it in the fridge for 30 minutes.
Heat the oven to 200°C/400°F/gas 6. Line the pie crust with baking paper and dried baking beans, then bake
for 20 minutes. Remove the paper and beans and return it to the oven for 5 more minutes. Allow the pie crust to cool, then pour in the melted chocolate and allow it to set.
For the filling, combine all the ingredients in a bowl, then pour the mixture into the pie crust. Reduce the oven to 180°C/350°F/gas 4 and bake the pie for 40–50 minutes, until the filling is just set (be careful not to overcook it as it can crack). When it’s cooked, let it cool, then leave the pie in the fridge overnight to chill.
When ready to serve the pie, sprinkle the granulated sugar over the surface and caramelise it with a cook’s blowtorch.