Preheat the oven to 350℉. Line 2 baking sheets with parchment paper and set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the egg.
In a separate bowl, mix together the hot chocolate mix, flour, and baking powder until fully combined.
Slowly mix the dry ingredients into the butter mixture until fully incorporated.
Using a cookie scoop, scoop the cookie dough onto the baking sheets.
Cut each marshmallow in half. Place the marshmallow sticky side down on each cookie and press down until the cookie flattens into a round disk.
Bake the cookies in the preheated oven for 10 minutes. Set your oven to broil and broil the cookies for around 30 seconds, or until the marshmallows are golden brown. Make sure you keep an eye on them so that they don't burn!
Allow the cookies to cool slightly before eating.