Combine graham cracker crumbs and butter. Press 1/4 of the crumbs into the bottom of 4 dessert cups, lining the sides and bottom of each cup. Fill cups with vanilla or chocolate ice cream. Shave curls of bittersweet chocolate over cups using a vegetable peeler. Thread marshmallows on to a metal skewer. Hold the skewer over a burner at high heat and toast the marshmallows. Top each cup with a toasted marshmallow and serve.