Whisk together flour, salt, baking powder, baking soda, and cornstarch and then gradually mix into butter mixture until just combined.
4 Add marshmallows, chocolate chips, and Graham crackers to the dough and mix on low until just combined.
5 Refrigerate dough for an hour.
6 Heat oven to 400 degrees. Scoop dough into balls and place on lined baking sheet.
7 Bake for 8 minutes then removed cookies from oven. Place additional marshmallows, Hershey's chunks (or additional chocolate chips) and Graham pieces on tops of cookies.
8 Return to oven for about 2 minutes or until cookie is done.
9 If some cookies have lost shape due to puffed marshmallows, you can reshape with a round cookie cutter before cooling.