Place flour and cocoa in a bowl. Mix. Add sugar. Mix. Add cubes of cold cubed butter. Mix with a pastry cutter or back of a fork until you get a sand like crumb.
Add water a little at a time combining as you go. You may need less water than indicated to form a ball of dough. Put into a small bowl and cover with plastic wrap or wrap with parchment paper and fold over so it is not exposed to air. Refrigerate for 2 hours.
Remove from fridge and roll out on a very lightly floured surface so it is the same thickness all the way across. Place in a pie pan.
You can add the filling to the uncooked crust and bake according to your recipe instructions. Or this is How to Blind Bake first and then add a cold filling once it's cooled.