Preheat your oven to 350°F (175°C). In a large mixing bowl, beat softened unsalted butter with granulated sugar and dark brown sugar until light and fluffy, about 2-3 minutes. Add egg and vanilla extract, mixing until creamy.

In a separate bowl, whisk together all-purpose flour, natural unsweetened cocoa powder, hot cocoa mix, baking soda, and salt until no lumps remain.

Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. If too thick, add 1-2 tablespoons of milk.

Cover dough tightly with plastic wrap and refrigerate for at least 2 hours.

Scoop out tablespoon-sized portions and roll into balls. Place on a lined baking sheet, spaced about 2 inches apart. Bake for 10 minutes, until edges are set.

Remove from oven, press a halved marshmallow into the center of each cookie. Bake for an additional 2 minutes.

Let cool on the baking sheet for a few minutes before transferring to a wire rack. Drizzle with melted semi-sweet chocolate once cooled.