1. Preheat your oven to 350°F and line baking sheets with parchment paper or silicone mats.

2. In a large bowl, cream the softened butter and both sugars together until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla until fully incorporated.

3. In a separate bowl, whisk together the flour, cocoa powder, hot cocoa mix, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. The dough will be thick and sticky. Beat in the milk until the dough comes together.

4. Cover the dough and chill it in the refrigerator for at least 2 hours, or up to 3 days.

5. Once chilled, scoop the dough into balls, about 1 tablespoon each, and place them on the prepared baking sheets, spacing them 2-3 inches apart. Bake for 10 minutes, then remove the trays from the oven and press the marshmallow halves, cut side down, onto each cookie. Return the cookies to the oven and bake for 2 more minutes.

6. Gently press down on the marshmallows with a spoon to help them adhere to the cookies. Allow the cookies to cool on the baking sheets for 10 minutes before transferring them to a wire rack to cool completely.

7. While the cookies are cooling, melt the chopped semi-sweet chocolate using a microwave or double boiler. Drizzle the melted chocolate over the marshmallow-topped cookies, allowing it to set at room temperature before serving or storing.