1Heat oven to 350°F. Grease 13x9-inch pan. Mix 2 cups of the cracker crumbs and the butter in medium bowl. Press on bottom of pan. Bake 10 minutes. Cool completely, about 20 minutes.
2Mix marshmallow creme and 1 cup chocolate topping in large bowl until smooth. Fold in whipped topping and 2 cups of the marshmallows. Spoon filling evenly over cooled crust.
3Sprinkle remaining 1/4 cup cracker crumbs and 1 cup marshmallows over top of dessert. Drizzle remaining 1 cup chocolate topping over top of dessert.
4Spray a piece of foil with cooking spray. Tightly cover dessert with foil, sprayed side down. Refrigerate at least 2 hours or freeze up to 3 days. Cut into squares.