Preheat oven to 350°.
Cream butter and brown sugar.
Scrape sides and add egg and vanilla. Mix well.
Sift together cornstarch, soda, salt, flour, and graham cracker crumbs. Gradually add the dry ingredients to the butter/sugar mixture while mixing slowly.
Mix until just combined. Over mixing the flour will result in stiff cookies. Gently fold in the marshmallow bits and mini chocolate chips.
Scoop into balls and place on a parchment paper lined cookie sheet. Use your fingers to press down the tops of the balls a little.
Bake at 350° for 8-10 minutes. To insure that the cookies stay chewy and soft, you’ll want to pull them when the cookies are no longer glossy on the top. Don’t wait until they are golden brown! They cookies will firm up as they cool.
Cool on the pan for 5 minutes and then transfer to a wire rack. Enjoy with a tall glass of milk.