Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Stir in the chocolate chips.
Drop rounded tablespoons of dough onto the prepared baking sheets, about 2 inches apart.
Bake for 10-12 minutes, or until the edges are set and the centers are still slightly soft.
Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Notes
For an extra milkshake flavor, you can add a splash of chocolate syrup to the dough. Ensure butter is truly softened for the best cookie texture.