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.
Mix in eggs one at time then stir in vanilla. With mixer set on low speed, slowly add in flour mixture and blend just until combined. Mix in 12 oz of chocolate chips.
Melt remaining 12 oz chocolate heat in a microwave on 50% in microwave in 30 second increments, stirring between increments until melted and smooth. 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).
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