Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl and set aside.
188 g all-purpose flour, 41 g unsweetened cocoa powder, ½ tsp baking soda, ¼ tsp table salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar and 65 g of the granulated sugar on medium-high speed until smooth and creamy.
115 g unsalted butter, 100 g dark brown sugar, 135 g granulated sugar
Add the egg, and vanilla and peppermint extracts and beat on high speed for 1 minute. Scrape the sides and bottom of the bowl and continue beating until well combined.
1 large egg, 1 tsp pure vanilla extract, ¼ tsp peppermint extract
Pour the dry ingredients into the wet ingredients. Add the milk and mix on low until combined into a thick cookie dough.
2 tsp whole milk
Rolls balls about 1 inch in diameter and roll them into the remaining 70 g of granulated sugar. Place on a large parchment paper lined baking sheet.
Make an indent in each of the cookies with your thumb, or the end of a wooden spoon. Push any cracks together with your hands.
Chill the shaped cookies for at least 2 hours in the refrigerator. Cover them if chilling for more than 3 hours.