Preheat oven to 350 degrees F. Lightly spray a 9x13 pan with nonstick cooking spray. Line with parchment paper and spray again lightly with nonstick cooking spray. Set aside.
Stir melted butter, brown sugar and granulated sugar until well combined. Add in egg, egg yolk and vanilla. Mix until lighter in color.
Add in the flour, baking soda and salt. Scrape the sides of the bowl and the bottom really well to ensure a smooth, well stirred batter. Fold in the graham cracker crumbs.
Divide dough into thirds and spread 2/3rds of the cookie dough into the bottom of prepared pan. Top with chocolate bar pieces and mini marshmallows. Spread remaining 1/3rd of cookie dough overtop of the chocolate and marshmallows. I find the easiest way to do this is to take a small about, flatten it in my palms and place overtop. There isn't enough dough to cover the entire pan, so its fine if a few marshmallows or chocolate pieces are peeking through.
Bake 15-20 minutes, rotating pan half way through baking to ensure even cooking.
Remove from oven and cool completely. Use parchment paper to remove from pan easily and cut into 24 even bars. Store in air tight containers for up to three days at room temperature or up to 5 days refrigerated.