Preheat oven to 350℉ and line baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, add the butter, sugar and brown sugar. Mix on low until combined then on high until light and fluffy.
Add the egg, milk and vanilla and mix until well-combined, scraping the bowl with a rubber spatula at least once halfway through mixing.
Add the flour, hot cocoa mix, cocoa powder, baking soda and salt and mix until incorporated.
Add the mallow bits and chocolate chips; stir until well-dispersed.
**Optional: Cover the dough and refrigerate for up to 36 hours.
Use a cookie scoop to evenly scoop the dough onto the prepared baking sheets.
Bake one sheet at a time for 8-10 minutes; do not overbake. The cookies will still be very soft in the center.
Press a few additional chocolate chips and mallow bits onto the cookies while they're still hot.
Let the cookies cool completely on the pan (the heat from the pan will continue to bake the cookies a bit).