Place the butter, brown sugar and granulated sugar in the bowl of a mixer. Beat until light and fluffy.
Add the egg, egg yolk and vanilla to the bowl and beat until well combined.
Add the baking soda, corn starch, salt, all purpose flour and graham cracker crumbs to the bowl. Stir on low speed until combined.
Add the marshmallow bits and approximately 2/3 of the Hershey bars to the bowl. Beat for 20 seconds or until just combined.
Cover the bowl of dough and let it chill for an hour.
Preheat the oven to 350 degrees F. Line a sheet pan with parchment paper or a nonstick baking mat.
Roll 1 inch balls of dough and place them 2 inches apart on the prepared pan. You will have to work in batches.
Bake for 8-10 minutes or until edges are set. Remove the cookies from the oven and quickly press a few pieces of chocolate and miniature marshmallows into the tops.
Repeat with remaining cookie dough. Let cool, then serve, or store in an airtight container for up to 3 days.