Preheat oven to 350 degrees F.
In the bowl of a standing mixer, add the room temperature butter, sugar, and brown sugar. Using the paddle attachment, cream until light and fluffy—about 3 minutes.
Slowly add in the egg, egg yolk, and vanilla then mix on low to combine, scraping down the bowl as necessary.
In a separate bowl, whisk together the salt, cocoa powder, flour, and baking soda. Add the dry ingredients to the butter mixture and mix on low, just until combined. Scrape down the sides of the bowl as needed to be sure there are no lumps of butter in your dough.
Once mixed, scoop and roll the cookie dough into roughly 24 1¼" balls and roll in the hot chocolate mix powder. Place about 2" apart on a baking sheet lined with parchment paper and flatten slightly with the palm of your hand.
Bake in a preheated oven for 3 minutes, then remove from oven and lightly press a large marshmallow onto the top of each cookie. Return to the oven and bake for about 3-5 more minutes, or until marshmallows are puffed but not browning and the edges of the cookies are set.
Remove cookies from the oven and place a Hershey's Kiss onto the top of each marshmallow while they are still very hot. Let the cookies cool for 2 minutes on the baking sheet before transferring to a cooling rack to cool completely. Enjoy!