STIR crumbs with butter in a medium bowl until mixture resembles wet sand. Fill the mould with mixture, pushing down with your fingers to get rid of any air pockets. Flatten the top. (This mixture will make 4 cookie cups.) Set the mould on a baking sheet. Chill in freezer until cups are very firm, 1 to 2 hours.
REMOVE cookie cups from mould. (This can be tricky. Luckily, the moulds are flexible so you can do some creative shimmy-ing to get the cups out.)
BRUSH melted chocolate on the inside of cooled cups. Return to fridge until chocolate is firm, about 5 min. Scoop marshmallow fluff into a microwave-safe bowl. Microwave just until fluff is a bit runny, 5 to 10 sec. Fill graham cups with fluff. If you have a brulee torch, torch the fluff for an authentic toasty s’mores taste. Enjoy!