Heat oven to 350°F. Grease 13x9-inch pan with shortening. In large bowl, beat all crust ingredients with electric mixer on low speed until coarse crumbs form. Press mixture in bottom of pan. Bake 10 to 12 minutes or until golden brown.
Remove partially baked crust from oven. Sprinkle marshmallows over crust. Return to oven; bake 1 to 2 minutes longer or until marshmallows begin to puff.
Remove pan from oven. Sprinkle chocolate pieces evenly over marshmallows. Drizzle warm ice cream topping over top. Cool completely, about 1 hour. Cut into bars.