1. Heat broiler. Break graham crackers into squares, and place on a rimmed baking sheet. Layer chocolate over a third of the graham-cracker squares; cover the remaining graham-cracker squares with marshmallows. Place under the broiler, and toast until chocolate is just melted and marshmallows are lightly browned, 30 seconds to 1 minute.

2. Remove from oven, and while still warm, transfer a marshmallow-covered cracker to each serving dish. Add a scoop of chocolate ice cream. Top chocolate ice cream with a chocolate-covered cracker, and add a scoop of vanilla ice cream. Top vanilla ice cream with another marshmallow-covered cracker, and drizzle with hot fudge sauce. Serve immediately.