In a large bowl, cream the butter, granulated sugar, and brown sugar together until fluffy and light in color (about 3 minutes).

Add the egg and vanilla extract. Beat until well combined, scraping down the sides of the bowl as needed.

In another bowl, whisk together the flour, cocoa powder, hot cocoa mix, baking soda, and salt.

Add the dry ingredients to the wet mixture and beat on low speed until combined.

Add the milk and beat until fully combined. The dough will be thick and slightly sticky.

Cover the dough tightly with plastic wrap and refrigerate for at least 2 hours (or up to 3 days).

Preheat the oven to 350°F (177°C) and line baking sheets with parchment paper or silicone mats.

Scoop the chilled dough into balls using a tablespoon and place them on the prepared baking sheets, leaving space between each.

Bake the cookies for about 10 minutes. Remove from the oven and gently press marshmallow halves onto each cookie.

Return to the oven and bake for an additional 2 minutes, then remove from the oven and press the marshmallows gently to flatten.

Let the cookies cool on the baking sheet for about 10 minutes before transferring them to a wire rack.

Melt the chocolate and drizzle it over the cooled cookies. Allow the chocolate to set at room temperature for 30-60 minutes.