Melt butter in 30 second intervals until melted. Add brown sugar and white sugar to the bowl and use an electric mixer to beat until everything is well combined.
Add in the egg+yolk and vanilla extract. Beat again.
Pour the all purpose flour, baking powder, salt, and ⅓ cup graham cracker crumbs to the batter. Mix again until cookie dough forms.
Add in ½ cup of mini marshmallows, chocolate chips, and fold together.
Layer ½ of the cookie dough into the bottom of the prepared pan. Break 1 Hershey’s bar into pieces and spread pieces in an even layer on the dough. Add the remaining marshmallows.
Add the remaining cookie dough on top of the chocolate/marshmallow layer. Bake for 22-25 minutes.
Remove from the oven and place the remaining Hershey’s Bar pieces onto the top while it’s still hot (they’ll get a little melty and it is DELISH).
Allow the casserole to cool before cutting into squares. Enjoy!