Preheat your oven to 375 degrees.
Combine the flour, cocoa, salt, and baking soda into a medium sized bowl and put aside.
Melt the butter and pour it into a large bowl or the mixing bowl from your stand mixer.
Add both of the sugars and cream them into the butter.
Then add both the egg and the egg yolk, the milk, and whatever extract you chose. Mix well to evenly distribute.
Then add the flour and cocoa powder slowly while the mixer is going so it doesn't go flying everywhere.
Then stir in the dark chocolate & mint chips.
Place the dough in the fridge for at least a half hour. Allowing the dough to chill will harden the fat (butter) so that the cookies won't spread as much in the oven. If you want your cookies to spread more than just skip this step and you will end up with a more crisp cookie.
Scoop the cookies onto a baking sheet with parchment paper on it, or a Slipat. I can usually fit about 6 cookies per sheet.
In my oven I bake them for about 10-12 minutes. It's a good idea to check the cookies after 5 minutes to gauge how long you should cook them. Since this cookies are dark already because of the cocoa it will be more of a challenge to tell when they are done. I always give some time for the first batch to cool to be sure I baked them the right amount of time (if not I adjust accordingly from then on). I typically get about 18 good-sized cookies out of this recipe. Enjoy!