In a double boiler or microwave safe bowl melt chocolate chips until smooth.
Spread half the melted chocolate over a silicone baking mat or lined baking sheet and put in the fridge for 15 minutes to set.
Microwave marshmallow fluff for 15-30 seconds until it can be easily stirred and spread over set chocolate. Leave a lip of about 1/2 an inch to allow the warm fluff to spread without overflowing onto the baking mat. Put into the fridge to set for about 15 minutes.
Pour remaining melted chocolate over the marshmallow fluff and immediately top with crushed graham crackers. Allow to set in the fridge for another 15-20 minutes before breaking into pieces and serving.
Store leftovers in an airtight container in the fridge.