1. Melt 4 T (1/2 stick) butter in medium saucepan over medium heat. Add cocoa powder and espresso powder. Stir until it forms a smooth paste. Cool for 15 to 20 minutes

2. In standing mixer with paddle attachment, mix remaining butter, sugar, salt, and cooled cocoa mixture on high speed until well combined and fluffy, about 1 min, scraping sides with rubber spatula. Add yolks and vanilla and mix on medium speed until thoroughly combined, about 30 sec. Scrape sides. With mixer on low, add flour in 3 additions, scraping in between each. Continue mixing until mixture balls up, about 5 more seconds.
Turn dough onto counter and refrigerate until firm in plastic wrap or parchment paper, either in 3 4-inch disks (if you are going to roll out and use a cookie cutter) or in a dough log, (about 2 inches in diameter and 12 inches long - totally easy method)
3. Preheat oven to 375 degrees. Roll out dough between two sheets of parchemnt paper to an even thickness of 3/16 inch. Peel parchment from one side and use cutters if desired, and put on parchment lined cookie sheet. Re chill if the dough becomes sticky. For the log, simply slice 1/4 inch and place on parchment-lined baking sheets.
4. Bake until cookies show slight resistance to touch, 10 to 12 minutes, rotating baking sheet halfway through baking time. Do not overcook or let edges darken, or they will taste burnt. Cool completely. Glaze or decorate if desired.

Cook's Illustrated (Sorry don't sue me)5 dozen60SO GOOD...can't get enough. It has two pauses in the beginning to cool and chill, so it starts up slow and you can blog in between! The dough log is so easy, I think I might start making these all the time instead of buying chocolate cookies. 2 1/2 sticks butter (softened to room temp)1/2 cup (about 2 oz) cocoa powder1 tsp instant espreso powder 1 cup sugar1/4 tsp salt2 large egg yolks1 Tbsp vanilla extract2 1/4 cups unbleached all-purpose flour