Dump flour, cocoa, sugar and salt in a clean and dry surface. Using your finger tips or a fork, whisk them gently to combine. Scatter the cold, cubed butter on top of the dry ingredients.
Incorporate the butter into the powders by pressing the butter with your finger tips, smashing them into the flour with your hands and pressing them against the surface until the butter and flour turn lumpy and coarse. ( You can also use a pastry blender, if you prefer). Gather them into a heap and make a well in the center. Add in the egg yolk and cold water into the well and by smashing, pressing method, incorporate the ingredients together until you can form them into a ball when pressed. This crust will be crumbly. If necessary add a tiny amount of water if it is too dry that you can't form it into a shape.
Gather the dough and wrap in plastic wrap. Refrigerate for at least half an hour. After resting, generously flour a work surface. Pat the dough to flatten it slightly and using a rolling pin, roll it out to a thickness of about 1/4 inch. If dough breaks, simply press it again to patch it up. Lift the dough with the help of a rolling pin and gently center it into a 9-inch pie or tart pan. If dough falls apart, mend them together, using scraps of the crust if necessary. Gently press the dough to fit into the edges of the pan. Cut the excess dough on the pan edges, then make the edges neat by pressing them with your fingertips. Refrigerate the pan to let it rest after the rolling and shaping. Preheat oven to 350 F.
Bake the crust for about 17-20 minutes until it looks dry and firm when touched. Let cool and set aside.