Begin by softening the butter at room temperature. Beat the softened butter with granulated sugar and dark brown sugar until fluffy and light in color, about 3-4 minutes. Mix in the egg and vanilla extract until well combined.

In a separate bowl, whisk together the flour, cocoa powder, hot cocoa mix, baking soda, and salt until thoroughly combined.

Gradually add the dry mixture to the butter-sugar blend, stirring gently. Add a splash of milk and mix until it comes together into a smooth dough.

Cover the cookie dough and chill in the refrigerator for at least 2 hours or up to 3 days.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop tablespoons of chilled dough and roll them into balls, placing them 2 inches apart on the baking sheet.

Bake the cookies for 10 minutes. They should be slightly underbaked with a soft puffed texture.

Remove the baking sheet and insert a halved marshmallow into the center of each cookie. Return to the oven for an additional 2 minutes.

Allow cookies to cool on the baking sheet for about 10 minutes. Melt the semi-sweet chocolate and drizzle it over each cookie before serving.