Beat the butter, granulated sugar, and brown sugar together on medium-high speed until fluffy and light in color, about 2–3 minutes. Add the egg and vanilla, then beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.
In another bowl, whisk the flour, cocoa powder, hot cocoa mix, baking soda, and salt together until combined.
Gradually add the dry ingredients to the wet ingredients. Beat on low until combined. The cookie dough will be thick and sticky. Finally, beat in the milk.
Chill the sticky cookie dough in the refrigerator for at least 30 minutes.
Preheat your oven to 350°F (175°C) and line large baking sheets with parchment paper.
Scoop and roll dough into balls, about 1 heaping tablespoon (25-26g) each. Place them 2–3 inches apart on the baking sheets.
Bake for 10-12 minutes. Remove the cookies from the oven and press a marshmallow half into the top of each cookie. Bake for an additional 2 minutes.
Remove from the oven and gently flatten the marshmallow using the back of a spoon.
Let the cookies cool on a wire rack completely.
Melt the chocolate and spoon it over each cooled marshmallow-topped cookie. Let it set at room temperature for 30-60 minutes.
Notes
Store cookies in an airtight container for up to a week or freeze for up to 3 months. Use vegan butter for a dairy-free option.