Line 15x10x1-inch baking pan with foil. Butter or spray foil with cooking spray. In large saucepan, melt chocolate chips and 1 cup of the butter over low heat, stirring frequently. Remove saucepan from heat. Stir in peanut butter until well blended. Spread half of mixture in pan. Freeze 10 minutes or until set. Place pan in refrigerator.