Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl. Set aside.

Using a hand mixer or a stand mixer fitted with paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium speed until creamy, about 3 minutes. Add the egg and vanilla extract and mix on high until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed.

Pour the dry ingredients into the wet ingredients, add the milk and food coloring, and then mix on low speed until everything is combined. The dough will be sticky. If you want a more vibrant hue, beat in more food coloring a little at a time. Add the chocolate chips and beat on low speed until just combined.

Cover and chill the dough in the refrigerator for at least 1 hour (and up to 3 days). I recommend just 1 to 2 hours, otherwise the cookies won’t spread much.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Scoop and roll cookie dough, about 1.5 Tablespoons (30–35g) of dough each. Arrange on the baking sheet about 3 inches apart. Bake for 11–13 minutes or until the edges appear set. Centers will look very soft.

Cool for 5 minutes on the baking sheets. Transfer to a wire rack to cool completely.

Cover and store leftover cookies at room temperature for up to 1 week.