Preheat your oven to 350°F (175°C). In a large mixing bowl, beat the butter, granulated sugar, and dark brown sugar until fluffy and light in color.

Add the egg and vanilla extract, mixing until creamy and well combined.

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

Gradually add the dry ingredients to the wet dough, mixing on low speed until just incorporated. Stir in milk to achieve a slightly sticky dough.

Chill the dough in the refrigerator for at least 2 hours.

Remove the dough from the fridge, scoop out tablespoon-sized portions, and roll them into balls. Place on a lined baking sheet.

Bake for 10 minutes until slightly underdone.

Press halved marshmallows onto the top of each cookie and return to oven for an additional 2 minutes.

Allow cookies to cool for about 10 minutes before transferring to a wire rack. Drizzle melted chocolate over the cookies.