Preheat the oven to 350°F (180°C). Line a baking sheet with parchment paper. Set aside.
Add the softened butter to the bowl of your stand mixer along with the white sugar and brown sugar and cream them together until smooth.
Add the egg and vanilla extract and beat in. Scrape down the sides of the bowl as needed and then mix again.
Sift the all purpose flour and cocoa powder into a bowl. Add the baking soda and salt and mix in.
Then add all the dry ingredients in with the wet ingredients and mix in by hand (don't use the electric mixer for this part) until crumbly.
Add the milk and mix into a thick cookie dough.
Add the chocolate chips or chopped chocolate chunks and fold in.
Roll the cookie dough into balls and place them evenly onto the parchment lined baking tray. Aim for around 20 cookies.
Place into the oven and bake for 10 minutes.
The cookies will be very soft but will firm up as they cool.
Place extra chocolate chips or chunks directly into the top of the cookies as they come out of the oven.
Allow to cool completely on the tray before moving them.