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1
Put the flour, cocoa, sugar, and salt on a smooth work surface, preferably a cool surface such as marble.
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2
Toss the ingredients together lightly with your fingertips, then scatter the butter pieces across the dry ingredients.
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3
Use your fingertips to work the butter into the flour mixture until it forms pieces the size of small peas.
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4
Then use a combination of techniques to work the butter further into the flour:
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5
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.
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6
Gather the mixture into a mound, make a volcano-like well in the center, and pour in the yolk and ice water.
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7
Use your fingers to break up the yolk and start moistening the dry ingredients.
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8
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.
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9
The dough will be crumbly and not really cohesive.
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10
Bring it together by smearing small portions of it across the work surface with the heel of your hand.
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11
Chilling the Dough: Gather the dough together and shape it into a rough square.
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12
Pat it down to compress it slightly, and wrap it in plastic.
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13
Chill until firm, at least 30 minutes.
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14
Storing: The dough can remain in the refrigerator for 3 days, or it can be wrapped airtight and frozen for a month.
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15
Thaw the dough, still wrapped, overnight in the refrigerator before rolling it out.
