PREHEAT broiler. Butter 13 x 9-inch baking pan.
BREAK graham crackers into small pieces; discard crumbs. Combine graham cracker pieces, 4 cups marshmallows and 1/2 cup morsels in large bowl.
HEAT remaining marshmallows, remaining morsels, butter and corn syrup in large saucepan over medium heat, stirring frequently until smooth. Remove from heat; stir in vanilla extract. Pour over graham cracker mixture; toss well to coat. Firmly press mixture into prepared pan.
BROIL for 1 to 2 minutes or until marshmallows are golden. Cut into bars.