Line a baking sheet with parchment paper and set aside.
In a microwave-safe bowl, melt the milk chocolate chips in 30-second intervals, stirring in between until fully melted and smooth.
Pour the melted milk chocolate onto the prepared baking sheet and spread it out evenly using a spatula.
In another microwave-safe bowl, melt the dark chocolate chips in the same manner until smooth.
Drizzle the melted dark chocolate over the milk chocolate, then use a toothpick or a small spatula to swirl the two chocolates together for a marbled effect.
Immediately sprinkle the graham cracker crumbs evenly over the chocolate layer, followed by mini marshmallows and mini chocolate chips.
Place the baking sheet in the refrigerator for 1 to 2 hours, or until the chocolate has completely hardened.
Once solid, remove the bark from the parchment paper and break it into pieces.
If desired, you can toast the marshmallows slightly with a kitchen torch for a campfire flavor before serving.
Notes
Toast the marshmallows for a campfire flavor if desired.