1. Preheat oven to 375 degrees Fahrenheit. Line two baking sheets with Gefen Parchment Paper.
2. In a large mixing bowl, combine butter, brown sugar, and granulated sugar. Beat for three minutes until creamy.
3. Add eggs and vanilla and beat well until incorporated.
4. Add flour, cornstarch, baking soda, and salt, mixing just until combined.
5. Add one and a half cups of the graham cracker pieces, one and a half cups of the marshmallows, milk chocolate chips, and semisweet chocolate chips, stirring just until dispersed throughout the dough.
6. Scoop two-to-three-tablespoon size balls of dough onto the parchment lined baking sheets, leaving room for the cookies to spread. Bake for 10 minutes until almost done, then remove from the oven and press four to five additional marshmallows on top of each cookie.
7. Return to oven and bake for one to two minutes until done. If desired, switch oven to broil and toast the marshmallows on top. But be careful as the small marshmallows can burn quickly.
8. Cool for two to three minutes on the pan before transferring to a wire cooling rack. Store in an airtight container on the counter for four to five days.