Heat oven to 350°F. Spray bottom of 13x9-inch pan with nonstick cooking spray. Arrange graham cracker squares in bottom of pan, breaking to fit if necessary.
Prepare brownie mix as directed on package, using oil, water and eggs. Carefully spread batter evenly over graham crackers. Bake at 350°F. for 28 to 30 minutes.
Remove base from oven. Sprinkle with marshmallows. Return to oven; bake an additional 1 to 2 minutes or until marshmallows begin to puff.
Remove bars from oven. Sprinkle evenly with chocolate pieces. Drizzle with warm ice cream topping. Cool 1 hour or until completely cooled. Cut into bars.