Before getting started, pre-heat your oven to 350ºF and line 2 13x9 baking sheets with parchment paper.
Start by whisking together the flour, baking powder, salt and hot cocoa mix in a large mixing bowl.
In another large bowl, cream together the brown sugar and butter until well combined and fluffy. I like to use my electric stand mixer with a paddle attachment for this step.
Next, add the egg and vanilla extract, beating 'til well combined.
Then gradually add the dry mixture a little bit at a time, beating 'til well combined in between each addition.
Finally, use a spatula to fold in the chocolate chips and marshmallow bits 'til they're evenly dispersed all throughout the cookie dough.
Now that your cookie dough is all mixed up, use a tablespoon or cookie dough scoop to scoop the dough out evenly onto the baking sheets that you prepared in the first step.
Bake the cookies for 8-10 minutes, or 'til firm and a toothpick inserted into the center of one of the cookies comes out clean. The middle should also be soft and puffy, and it should spring back when lightly pressed into.
That's it! Serve and enjoy!