Step 1

Preheat oven to 350 degrees Fahrenheit (177° Celsius, Gas mark 4).

Step 2

In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt. Set aside.

Step 3

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar. Scrape down the sides of the bowl. Add egg yolk and vanilla and mix until well combined. Scrape down sides of bowl again.

Step 4

Add dry ingredients to bowl. Mix on low speed to incorporate. Scrape paddle attachment and sides of bowl. Turn speed to medium and mix just until dough pulls together into a ball.

Step 5

Turn dough out onto a large rectangular piece of parchment paper. Flatten dough into a rough rectangle. Using a rolling pin, roll dough between parchment layers until ⅛" thick (3.18mm). Slide parchment covered dough onto an upside down baking sheet and chill in the fridge for 30 minutes.

Step 6

Remove dough from fridge and place on a flat work surface. Peel the top layer of parchment off the dough. Replace the parchment, flip the dough over, and peel the second layer off. Discard it. Use a cookie cutter to cut the dough into desired shapes (I used a 2½ inch - 6.3cm - square cookie cutter). Place the cookies 1 inch apart on a parchment lined baking sheet.

Step 7

Bake each sheet of cookies on center rack of oven for 10 minutes (for chewier cookies) or 12 minutes (for crispier cookies). Cool on sheet for 10 minutes before transferring to a cooling rack.