Prepare the Cookie Dough : Preheat your oven to 350°F (177°C) and line baking sheets with parchment paper or silicone baking mats. In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar using a hand or stand mixer on medium speed until smooth and creamy. Add the egg and vanilla extract, mixing until fully incorporated. In a separate bowl, whisk together the all-purpose flour, cocoa powder, hot cocoa mix, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms. Add the milk and mix until the dough is smooth.

Bake the Cookies : Using a cookie scoop or tablespoon, portion out the dough into 1.5-tablespoon-sized balls. Place them on the prepared baking sheets about 2 inches apart. Bake for 8–9 minutes, or until the edges are set but the centers are still soft. Do not overbake.

Add the Marshmallow Surprise : Remove the cookies from the oven and immediately press one marshmallow half (cut side down) into the center of each cookie. Return the cookies to the oven and bake for an additional 2 minutes, just until the marshmallows puff slightly.

Melt the Chocolate : While the cookies cool on the baking sheet, melt the semi-sweet chocolate in a microwave-safe bowl in 20-second increments, stirring after each interval, until smooth. Drizzle the melted chocolate over the cookies, covering the marshmallows and part of the cookie for a rich, glossy finish.

Cool and Serve : Allow the cookies to cool completely on the baking sheets so the chocolate can set. Serve with a cup of hot cocoa or enjoy them as a stand-alone treat!