Preheat the oven to 350°F. In the bowl of a stand mixer, beat together the sugars, butter, and eggs until smooth.
In a separate bowl, stir together the flour, salt, baking soda, baking powder, and hot cocoa powder.
Slowly add the dry ingredients to the bowl on the stand mixer, about ½ cup at a time, until a thick dough has formed. Add the mini semi-sweet chocolate chips and mini marshmallows (if using) and stir until combined. The cookie dough shouldn’t be very sticky.
Roll the dough into one-inch round circles, arrange on a baking sheet lined with parchment paper, then use the bottom of a glass to flatten the dough to about ¼ to ½ inch thick.
Bake the cookies for 8-10 minutes or until the cookies have started to crack across the top. Let the cookies cool completely.
While the cookies cool, use a hand mixer to whip together the marshmallow fluff, vanilla extract, and confectioners' sugar. Add heavy cream if the frosting is too dry, and more confectioners' sugar if it’s too thin. Aim for a toothpaste-like consistency.
Pipe or spread the tops of the cooled cookies with the marshmallow frosting. Serve on a cold day and enjoy!