LINE 8- or 9-inch-square baking pan with foil leaving an overhang on two sides. Spray foil with nonstick cooking spray.
HEAT 6 cups marshmallows, butter and salt in large, heavy-duty saucepan over medium- low heat, stirring frequently, for 5 to 10 minutes, until smooth. Remove from heat. Add 1 cup morsels; stir until melted.
WORKING QUICKLY, stir in graham cracker pieces, remaining 2 cups marshmallows and 3/4 cup morsels. Spread mixture into prepared baking pan with greased spatula, pressing down lightly. Cool for 2 hours or until set. Lift from pan; peel off foil. Cut into squares with serrated knife.