Before getting started, preheat your oven to 350ºF. Then line a cookie sheet with parchment paper or one of these reusable silicone baking mats.
Start by whisking together the flour, baking powder and hot chocolate mix in a medium mixing bowl.
In a separate large mixing bowl, cream together the butter and brown sugar 'til well combined, light and fluffy.
Then add an egg and the vanilla extract, beating into the mixture until well combined.
Gradually add the dry mixture to the wet mixture, beating everything together 'til fully incorporated.
Add the chocolate chips and marshmallow bits, stirring with a rubber spatula 'til evenly dispersed throughout the cookie dough.
Finally, use a cookie scoop or tablespoon to scoop the dough onto the baking sheet that you prepared in the first step.
Bake your hot chocolate cookies at 350ºF for 8-10 minutes, or 'til a toothpick inserted into the center of one of the cookies comes out clean.
Last but not least, use a cookie spatula to transfer them to a wire cooling rack. Once cooled, serve and enjoy with a piping hot cup of coffee or tea!