Preheat oven to 350 degrees. In a mixing bowl whisk together flour, cornstarch, baking soda and salt for 20 seconds, set aside. In the bowl of an electric stand mixer fitted with the paddle attachment, blend together butter, brown sugar and granulated sugar until creamy, about 30 seconds - 1 minute.
Shape dough into 2 Tbsp balls each and set on Silpat or parchment paper lined baking sheets, spacing 2-inches apart. Bake in preheated oven 9 - 12 minutes. Remove from oven and allow to cool completely.
Spread chocolate over the bottom side of half of the cookies. Toast marshmallows over a fire pit or over the stove burner rotating occasionally until golden brown (if using the stove burner be very careful that the marshmallows don't touch the burner, also I recommend using the shorter roasting sticks to toast over the stove).
Place 2 marshmallows over chocolate covered cookies then top with another cookie and serve immediately (you can make the cookies ahead of time, just add chocolate and marshmallows when serving for best results).