Preheat oven to 350°F. Prepare popcorn according to package directions. Remove all unpopped kernels. Place popped corn, graham crackers and marshmallows in large bowl; set aside.
Melt chocolate morsels in small saucepan over medium-low heat, stirring frequently. Pour over popcorn mixture. Mix thoroughly with spoon sprayed with cooking spray. Transfer to 13x9-inch baking dish sprayed with cooking spray.
Bake 10 minutes; remove from oven. Press down firmly on popcorn mixture with back of large spoon sprayed with cooking spray, re-spraying spoon as necessary. Refrigerate 30 minutes or until completely cooled. Cut into 18 squares to serve. Store in tightly sealed container up to 3 days.