In a small bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium-high speed for 2-3 minutes until light and fluffy.
Add in the egg yolk, milk, and vanilla, and mix until fully combined.
Add in the dry ingredients and mix until jut combined.
Shape the dough into a disk, wrap tightly with plastic wrap, and chill in the fridge for 1 hour or overnight.
On a lightly floured surface, roll out cookie dough to ¼" thick. Rolling the dough out between two pieces of floured parchment paper also helps to prevent the dough from sticking.
Cut out 2-inch circles and freeze the cutout dough for about 10 minutes. Repeat until all of the dough is used.
While the cutout dough is chilling in the freezer, preheat the oven to 350 degrees F (180 degrees C) and line two baking sheets with parchment paper.
Place the cut out cookie dough on prepared baking sheets about an inch apart and bake the cookies for about 10 minutes. Allow cookies to cool completely on the baking sheet.