PREHEAT oven to 350° F.
LINE 13 x 9-inch metal baking pan with foil leaving an overhang on two sides. Spray foil with nonstick cooking spray.
PLACE 15 graham cracker halves into bottom of pan, overlapping slightly. Break remaining graham cracker halves into 1/2-inch pieces; set aside.
COMBINE brownie mix, water, oil and eggs in medium bowl until blended. Stir in 1 cup morsels. Spread over graham crackers in pan.
BAKE for 25 to 30 minutes or until wooden pick inserted into center comes out still slightly sticky; remove from oven.
SPRINKLE remaining graham cracker pieces and marshmallows over warm brownies. Broil for 30 to 45 seconds or until marshmallows are light brown. Watch carefully as browning occurs very fast! A handheld kitchen butane torch can be used as well. Remove from oven to wire rack. Sprinkle immediately with remaining 1 cup morsels.
COOL for at least 1 hour at room temperature. Lift out by foil edges to cutting board. Carefully remove foil. Cut into bars with wet knife. Store in tightly covered container.