In a medium bowl, Sift together the flour, cocoa powder, sugar and salt.
Rub butter into the flour mixture with a pastry blender or your fingers until the mixture resembles coarse breadcrumbs.
Stir in the beaten egg and 2 tablespoons of water and bring it together to form a dough. Transfer the mixture to a work surface and briefly knead the dough just until it comes together. (add additional water if needed)
Shape into a disc, wrap well and refrigerate for at least two hours before rolling (and up to 2 days). It can also be frozen for up to 6 weeks.
Recipe Notes
You can use either regular or Dutch-processed cocoa for this pastry. Check out our Ultimate Guide to the Different Types of Chocolate to understand the nuances of the varieties of cocoa and chocolate used in baking.
You can mix this dough in a food processor if you wish. First, combine the dry ingredients, then pulse in the butter until crumbly, followed by the egg and water until just combined. Briefly knead the mixture into a dough on a work surface, then wrap well and refrigerate.
For recipes that require blind baking, fill a lined tart pan with parchment and pie weights and bake at 375°F (190°C) for 15 minutes, then remove the parchment and weights and continue baking until done, about another 5 to 10 minutes.