In a medium bowl, combine flour, cocoa powder, baking powder, and salt. Set aside.
In the bowl of a standing mixer with the paddle attachment or a using a handheld mixer, beat together butter, brown sugar and granulated sugar for about 1 minute.
Add in the milk, food colouring, and vanilla extract and mix until combined, scraping along the sides of the bowl.
Mix in the dry ingredients until just combined (the dough will be sticky), then fold in the chocolate chips.
Cover the bowl with plastic wrap so that it's airtight, then place the dough in the fridge to chill for 2 ½ hours.* (see notes)
Once the dough has chilled, preheat the oven to 350°F and line 2 baking sheets with parchment paper. Form the dough into 1.5 tablespoon-sized balls and place on the baking sheets, leaving room for them to spread a bit.
Bake one sheet at a time for between 9-12 minutes. Be sure to place the second tray of prepared dough into the fridge while the first batch of cookies is baking so that the dough doesn't soften.
When the cookies are done, the edges should look set and the tops will be slightly cracked. Remove the cookies from the oven and leave them on the baking sheet for 5-10 minutes, then transfer to a wire rack to cool completely.