Follow the package instructions to prepare the chocolate cake mix. Mix in 1 cup of chocolate chips until well combined.
Grease the bottom and sides of your 7QT slow cooker with non-stick spray or butter.
In the slow cooker, combine graham cracker crumbs, melted butter, and sugar. Mix and gently press down.
Spread the chocolate cake batter over the graham cracker crumbs.
Cook on high for 2 hours.
Remove the lid and sprinkle on marshmallows and the remaining chocolate chips.
Cover and cook until the marshmallows and chocolate are melted, around 30 minutes.
Serve right away and enjoy the deliciousness!