Preheat Oven & Prep Pan: Preheat your oven to 180ºC (356ºF). Line a baking sheet with parchment paper and set aside.
Combine Dry Ingredients: In a medium size mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda and salt. Set aside.
Cream Butter and Sugar: In a large mixing bowl, use electric beaters or a stand mixer with the paddle attachment, add the butter and beat it briefly to break it up, then add sugar and cream together on medium-high speed for 1 minute until light and creamy.
Incorporate Egg: Add the egg and beat for 15 seconds on medium speed.
Add Dry Ingredients & Milk: Add the flour cocoa mixture along with the milk and beat on low speed just until the dry ingredients are incorporated and a soft crumbly dough forms, ensuring not to overmix to keep the cookies tender.
Chill the Dough: Place the bowl with the cookie dough inside into the refrigerator to chill for 20 minutes.
Shape Cookies: Scoop one tablespoon of the dough (about 25g) and roll each scoop into a ball and flatten with the palm of your hand on the prepared baking sheet. Repeat with the remaining cookie dough, leaving a gap between each cookie.
Bake: Bake for 10-12 minutes, or until the cookies are set and peel away easily from the parchment paper.
Cooling: Remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.