In food processor with the metal blade, add flour, cocoa powder, sugar, salt, and baking soda. Pulse a few times to combine thoroughly. Dollop the butter around the dry ingredients and pulse a few more times until large chunks begin to form. Mix milk and vanilla together in a small bowl. With the mixer running, pour the milk mixture into the feed tube in a slow even stream until the cookie crumb mixture starts to form together and ball up on the sides of the bowl.

Transfer the cookie dough to a work surface and knead it a couple of times to fully incorporate all of the ingredients. Divide the dough in half and shape each half into a long and narrow log, about 1-1/4 to 1-3/4 diameter. Depending on how large you want. Wrap each log in plastic wrap and refrigerate for at least 2 hours or up to a week.

Preheat oven to 350° F. Line 3 baking sheets with parchment paper. About 1/3 of the log for each sheet.

Slice the dough logs into approximately 1/8 thick slices and lay the dough rounds on the baking sheets about 1 inch apart. 1-1/4 diameter logs made 24 cookies on each of 3 baking sheets.

Bake for 10-12 minutes, rotating pans from back to front and top to bottom halfway through the baking time. The cookies will be finished baking about 1-1/2 minutes after the puffed-up dough deflates so try to keep an eye on them. Cool the cookies on the baking sheets on wire racks for 5 minutes then transfer the cookies to the racks to cool completely - they will crisp up as they cool.

Storage: The cookies will keep for up to a week at room temperature if kept in an airtight container. They freeze really well too.

NOTES:

If you plan to use these cookies in place of store-bought chocolate wafer cookies for a pie crust like chocolate cheesecake, cut out 1 to 2 tablespoons of the melted butter in the crust recipe. These chocolate wafer cookies contain a fair amount of butter and too much added butter in the ground up cookies crumbs will yield an absurdly wet mass of crumbs.

Nine ounces of these cookies (or about 2/3. of the recipe) will yield as much as 1 package of store-bought wafer cookies contain.

adapted from Pure Dessert by Alice Medrich via Smitten Kitchen