In a medium bowl, whisk together the flour, baking soda, salt, and cocoa powder until well combined.

In a stand mixer, beat together the softened butter, melted chocolate chips, brown sugar, and granulated sugar until smooth and creamy. Then add the egg, vanilla extract, and milk, mixing until fully incorporated.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.

Wrap the cookie dough tightly in plastic wrap and chill in the refrigerator for at least 2 hours.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Once chilled, roll the dough into 1-inch balls and coat each ball with Christmas sprinkles. Arrange on the baking sheets.

Bake the cookies for 12-13 minutes, or until edges are set and centers remain soft. Allow to cool on the baking sheet before transferring to a wire rack.