In a mixer bowl, beat together butter, both sugars and salt on medium-high speed for 3-4 minutes, or until pale, thick, and fluffy.

Add in the eggs one at a time, stopping to scrape down the bowl between the two. Mix in the vanilla extract and continue to mix.

While it mixes, whisk together flour, cornstarch, baking soda, cocoa powder, and hot chocolate mix in a separate bowl.

Slow speed of mixer to low and slowly add in dry ingredients until well incorporated.

Fold in the chocolate chips and carefully swirl in the marshmallow fluff.

Line two baking sheets with parchment paper. Use an ice cream scooper to portion out 6 cookies on each sheet then if you desire, top each cookie with an additional teaspoon of marshmallow fluff. There will be extra dough, so you can either bake in batches or refrigerate/freeze the dough for later.

Place both baking sheets in the freezer and chill for 30 minutes.

Preheat the oven to 350 degrees.

Once chilled, bake the cookies for 11-13 minutes. Remove the pan from the oven and allow the cookies to remain on the pan for 5-10 minutes.

Transfer the cookies to a wire rack to cool completely, then serve!