In a large bowl, whisk together the flour, cocoa powder, sugar, and salt. Add in the cubed butter. Using your fingers, rub the butter into the dry ingredients until you have pea-sized pieces of butter remaining. In a small bowl or a cup, whisk together the egg white and the vanilla, add to the flour and butter mix. Stir until the dough begins to come together. You may need knead it a bit until you can form the dough into a ball. Flatten into a disc and wrap in plastic wrap. Let rest in the refrigerator for 1 hour.