Preparation / Directions:
To mix the dough in the food processor, combine the flour, sugar, cocoa, salt, and baking powder in the work bowl and pulse several times to mix. Add the butter to the bowl. Pulse to mix in completely. When it is incorporated correctly there should be no visible chunks of butter, but the mixture should remain cool and powdery.
Beat the egg with a fork and add to the bowl. Pulse until the dough forms a ball. If the dough does not easily form a ball, add 1/2 teaspoon water, repeating if necessary, until it does.
Remove the dough from processor, press into a disk and wrap in plastic. Chill until firm or for up to several days.
To mix the dough by hand, combine the dry ingredients in a mixing bowl and stir several times to mix. Rub the butter in, mixing it gently with fingertips, until it becomes a fine crumble. No visible pieces of butter should remain, but the mixture should stay cool and powdery. Beat the egg with a fork and stir into the dough mixture. Continue stirring until the dough holds together in a mass. Wrap and chill as above.
To mix the dough in a heavy-duty mixer, combine the dry ingredients in mixer bowl with paddle attachment. Beat on the lowest speed until combined. Add the butter. Beat on a low speed until the butter is absorbed but mixture is still powdery, about 1 minute. Beat the egg in a small bowl and add to the mixer bowl. Beat on a low speed until dough masses around paddle, about 30 seconds. Wrap and chill as above.