Preheat your oven to 350°F (175°C). In a large mixing bowl, beat the butter with the granulated sugar and dark brown sugar until light and fluffy, about 3-4 minutes. Add the egg and vanilla extract, mixing until smooth.

In a separate bowl, whisk together the flour, cocoa powder, hot cocoa mix, baking soda, and salt. Stir until dry mix is ready.

Gradually add the dry mix to the wet mixture, folding gently. If too dry, add a splash of milk. Cover and refrigerate for at least 2 hours.

Scoop and roll dough into balls, spacing them on a parchment-lined baking sheet. Bake for 10 minutes, checking for a slightly underdone appearance.

Press a halved marshmallow into the center of each cookie. Return to oven and bake for an additional 2 minutes.

Let cookies cool before drizzling melted semi-sweet chocolate over them.