Preheat your oven to 350°F (175°C). In a large mixing bowl, beat together the butter and granulated sugar until light and fluffy.

Add in brown sugar, egg, and vanilla extract, mixing until creamy.

In another bowl, whisk together flour, cocoa powder, hot cocoa mix, baking soda, and salt.

Gradually stir the dry mixture into the wet ingredients until just combined.

Chill the dough for at least 2 hours before baking.

Scoop the dough onto a lined baking sheet and bake for approximately 10 minutes.

Immediately press a halved marshmallow onto each cookie and return to the oven for an additional 2 minutes.

Allow to cool before drizzling melted chocolate over the cooled cookies.