Preheat oven to 400°. Line baking sheets with parchment paper or silicon mats.

Break graham cracker sheets in half. 

In a medium bowl, whisk together flour, baking soda, salt and cornstarch and set aside.

In a stand mixer, beat together the brown sugar, white sugar, 1/2 cup of butter and the butter flavored shortening, using the paddle attachment. Cream together for 2 minutes. Add the eggs one at a time, mixing well after each addition.

Add the flour mixture slowly. Mix until just blended.

Add the chocolate chips to the dough and mix. Add the marshmallows to the dough and mix only slightly until marshmallows are mixed in. Avoid over mixing at this point as it will break the marshmallows apart.

In a microwave safe dish, melt the other 1/2 cup of butter.  

Dip the graham crackers in the melted butter and arrange on the lined baking sheet about three inches apart.

Using a cookie scoop or spoon, round dough into a ball and place the dough on top of each graham cracker. For large cookies, make a dough ball about the size of a golf ball.

Bake for 8-10 minutes or until edges are lightly golden brown. Let cool on pan for 5 minutes before transferring to wire rack to cool completely.