Sift the flour, sugar, cocoa, and salt into a large bowl. With your fingers, incorporate the butter pieces into the dry ingredients until the mixture resembles coarse crumbs. Mix in the egg to make a soft dough. Flatten the dough into a disk, wrap tightly in plastic wrap, and place in the refrigerator to rest for at least 1 hour before using.
Yield: 1 (9 or 10-inch tart crust)