Put the flour, cocoa, sugar, and salt on a smooth work surface, preferably a cool surface such as marble.
Toss the ingredients together lightly with your fingertips, then scatter the butter pieces across the dry ingredients.
Use your fingertips to work the butter into the flour mixture until it forms pieces the size of small peas.
Then use a combination of techniques to work the butter further into the flour:
Break it up with your fingertips, rub it lightly between your palms, and chop it with the flat edge of a plastic or metal dough scraper.
Gather the mixture into a mound, make a volcano-like well in the center, and pour in the yolk and ice water.
Use your fingers to break up the yolk and start moistening the dry ingredients.
Then, just as you did with the flour and butter, toss the ingredients with your fingers and use the dough scraper to chop and blend it.
The dough will be crumbly and not really cohesive.
Bring it together by smearing small portions of it across the work surface with the heel of your hand.
Chilling the Dough: Gather the dough together and shape it into a rough square.
Pat it down to compress it slightly, and wrap it in plastic.
Chill until firm, at least 30 minutes.
Storing: The dough can remain in the refrigerator for 3 days, or it can be wrapped airtight and frozen for a month.
Thaw the dough, still wrapped, overnight in the refrigerator before rolling it out.