Stir the flour, cocoa powder and baking soda together in a small bowl.
Use a mixer to cream the butter, sugar and salt together. Run it on medium speed for about 3 minutes, scraping down the bowl periodically.
Blend in the vanilla on low speed and then add the egg white and mix 1-2 minutes.
Turn the mixer off and add half the flour mixture and blend; then add the other half and blend just until all the dry ingredients are mixed in.
Turn the dough onto your work surface and divide it in half.
Roll out each half of the dough to about 1/8", sandwiched between two sheets of parchment paper. Stack these on a cookie sheet and freeze an hour (or refrigerate for two hours).
Preheat the oven to 350 F with the racks set so the oven is divided in thirds.
Line the cookie sheets with parchment paper.
Peel each piece of paper away from the dough slab and lay the slab back on one of the pieces to cut the cookies out. Place on the lined cookie sheet about an inch apart and sprinkle with sugar. (I ended up dipping them into a plate of sugar, which let me get the sugar on more uniformly.)
Bake 15-17 minutes, rotating the pans halfway through. (If you're baking two cookie sheets at once rotate top to bottom and front to back.)
While the first set of cookies are baking, roll the scraps back together and roll out again, chilling the dough before cutting out the rest of the cookies.
Let the cookies cook on the cookie sheets for 2 minutes before moving them a wire rack to cool completely.
You can freeze cut out (unbaked) cookies and bake them straight from the freezer, adding another minute or so to the baking time.