In a large mixing bowl, beat the softened butter and both sugars with an electric mixer for 2–3 minutes, until creamy and fluffy. Beat in the egg and vanilla; scrape down the bowl.

Sift the flour, cocoa, baking soda, and salt directly over the wet mixture. Mix on low just until combined. Mix in the milk, then fold in the chocolate chips. The dough should be thick, sticky, and cohesive, not loose crumbs.

Cover and refrigerate for at least 3 hours. Let the chilled dough stand at room temperature for 10 minutes before scooping.

Preheat the oven to 350°F (175°C). Line baking sheets with parchment. Scoop rounded 1½-tablespoon portions (about 35–40 g), shape into tall balls, and space 2–3 inches apart.

Bake for 11–12 minutes, until the edges are set and the centers still look soft. Cool on the baking sheet for 5 minutes before transferring to a rack. Repeat with the remaining dough, keeping it chilled between batches.