1. Combine butter, marshmallows, and salt in a large saucepan. Cook over medium-low heat, stirring frequently, until smooth. Turn off heat, and add cereal. Stir into the marshmallow mixture until thoroughly combined, scraping all the mixture from bottom of pan.

2. Coat an 8-by-3-inch square professional baking pan with cooking spray. Cut out an 8-by-20-inch sheet of waxed paper. Set it in pan, with excess extending from two sides. Coat with nonstick spray. Spoon in mixture; coat palms of hands with cooking spray, and press firmly into pan, making sure that there are as few air pockets as possible. Be sure to press into corners of pan. Let cool.

3. Remove mixture from pan by lifting the overhanging waxed paper. Using a serrated knife, cut into 2-inch cubes. Stack cubes, as desired, to form a castle.