Add the flour, cocoa and icing sugar to a large bowl (or stand mixer bowl).
Add the chopped butter and either by hand (using the rub in method) or with the help of a stand mixer, mix the ingredients together. See my main post for further information. When the ingredients resemble a crumble-like consistency, it is ready. Don't overwork at this stage; it's perfectly fine to have large pieces of butter.
Add the egg yolk and mix in. Your ingredients will start to pull together as a dough.
If necessary, add one or two tablespoons of cold water. However, before doing so, make sure the egg has been fully incorporated. Your dough should be smooth and neither sticky nor too dry (where it falls apart). If you do need a little water, add just a touch at a time.
Shape the pastry into a rough ball and cover with clingfilm.
Place in the fridge for at least one hour.
When the dough is ready, pre-heat the oven to 175℃ (350℉).
Roll out your dough to around ½ cm (¼ ") - see note 2.
Using a round cutter slightly bigger than your cases or moulds, cut out 14 circles - see note 3.
Place each circle carefully into each case. Make sure you place it evenly in the tin, and allow gravity to do its work (rather than forcing the pastry in).
Gently push the pastry against the sides and bottom.
Place back in the fridge for 15 minutes.
Before placing the tartlet cases in the oven, prick all over the bottoms with a fork.
Place your tartlet cases onto a large oven tray, and then bake for 15 minutes in the lower part of the oven.
Remove from the oven. It's possible that the middles of the tartlets have puffed up (especially if the pastry is quite thick). If so, just gently push down with the back of a spoon while still hot.
Allow to cool completely and then move to the fridge. Let them rest for several hours before filling them. Alternatively pop them in the freezer until you need them. Enjoy!