Cream together the butter, granulated sugar, and brown sugar in a large bowl until the mixture is light and fluffy, about 2-3 minutes.

Incorporate the egg and vanilla extract, mixing until everything is well combined to form a smooth and velvety dough.

Whisk together the flour, cocoa powder, hot cocoa mix, baking soda, and salt in a separate bowl.

Combine the dry mixture with the wet ingredients. Mix just until incorporated, then stir in the milk.

Chill the dough by covering it and placing it in the refrigerator for at least 2 hours.

Preheat your oven to 350°F (177°C) and line baking sheets with parchment paper.

Roll the chilled dough into balls, measuring about 25-26 grams each, and place them on the baking sheets.

Bake the cookies for about 10 minutes, then press a halved marshmallow onto each cookie quickly.

Return the cookies to the oven for an additional 2 minutes to allow the marshmallows to puff.

Cool the cookies on the baking sheet for 10 minutes before transferring them to a wire rack.

Melt the chopped semi-sweet chocolate in a double boiler or microwave until smooth.

Spoon the melted chocolate over each marshmallow top and allow it to set for 30-60 minutes.