Beat the softened unsalted butter with granulated sugar and dark brown sugar until light and fluffy, about 3-5 minutes.

Add the egg and pure vanilla extract; mix until incorporated.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, hot cocoa mix, baking soda, and salt.

Gradually add the dry mixture to the wet mixture, stirring gently until combined. Add milk if too thick.

Cover the dough and chill in the refrigerator for at least 2 hours.

Preheat the oven to 350°F (175°C) and scoop dough into balls on a baking sheet.

Bake for approximately 10 minutes until slightly underdone.

Press halved marshmallows into the center of each cookie and bake for an additional 2 minutes.

Let cookies cool, then drizzle melted chocolate over the top.