In a bowl, mix flour, hot chocolate mix, dark cocoa powder, salt, and baking soda. Set aside.
In a large bowl, cream softened butter with white and light brown sugar until pale in color. Add the egg and vanilla, and beat until just combined.
Add the dry ingredients to the butter mixture and mix until combined.
Stir in chocolate chips and mini marshmallows.
Using a small ice cream scoop, take out a portion of cookie dough and place it on a pan that can fit in your fridge. Chill the cookie dough balls in the fridge for 30 minutes to an hour.
Meanwhile, preheat the oven to 350°F. Line one or two baking sheets with parchment paper.
Place the chilled cookie dough balls on the prepared baking sheets, spaced 2 inches apart.
Bake for 8-14 minutes, rotating the pan halfway thru baking. Let the cookies cool on the pan for 2 minutes before transferring them to a cooling rack.