Prepare the Dough:

• In a large bowl, beat the softened butter, granulated sugar, and brown sugar with a handheld or stand mixer on medium-high speed until light and fluffy (about 2–3 minutes).
• Add the egg and vanilla extract, beating on high speed until combined. Scrape down the sides and bottom of the bowl as needed.
• 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.
• Beat in the milk to slightly loosen the dough.
• Cover the dough tightly and refrigerate it for at least 2 hours and up to 3 days.

2. Bake the Cookies:

• Preheat the oven to 350°F (177°C) and line large baking sheets with parchment paper or silicone baking mats.
• Scoop the chilled dough into balls, about 1 heaping tablespoon each (25–26g), and roll into balls. Place them 2–3 inches apart on the baking sheets.
• Bake for 10 minutes. Remove from the oven and gently press a marshmallow half into the top of each cookie.
• Return the cookies to the oven and bake for another 2 minutes, until the marshmallows are softened.

3. Flatten the Marshmallows:

• After removing the cookies from the oven, use the back of a spoon to gently press the marshmallows, slightly flattening them. Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.

4. Add the Chocolate Topping:

• In a microwave-safe bowl, melt the chopped chocolate in 20-second intervals, stirring after each, until smooth and glossy.
• Spoon or drizzle the melted chocolate over the marshmallows on the cookies.
• Allow the chocolate to set before serving.