Line a baking sheet with parchment paper. With a hand mixer, cream the butter, granulated sugar, and brown sugar in a large mixing bowl.
Add the vanilla extract and then add one egg at a time while continuously mixing. Add the baking soda, salt, cocoa powder, and hot cocoa mix into the bowl. Then, gradually add the flour, making sure to scrape the sides of the bowl to incorporate all the ingredients.
Finally, fold in the chocolate chips. Using a cookie scoop or your hands, measure the dough into 3 tablespoon balls. Then flatten the dough balls with your hands and top each one with a marshmallow. Cover the entire marshmallow with cookie dough and roll it back into a ball. Once all the dough balls have been rolled, place them on the baking sheet and in the fridge for 20-30 minutes to chill.
Preheat the oven to 350℉ while the dough is chilling. Once the cookie dough is chilled, place the cookies on the parchment paper-lined baking sheet, 6 at a time, making sure to space them about 2 to 3 inches apart.
Bake for 10-13 minutes. They're ready when the outside of the cookies are set but the inside is still soft. It's normal for marshmallows to be showing through the cookies. While the cookies are baking, put the mixing bowl you'll use for the frosting in the freezer or fridge so it's nice and cold when you're ready to make the frosting.
Allow the cookies to cool for about 5-10 minutes then transfer to a wire rack to let them cool longer. While the cookies are cooling, make the whipped cream frosting.