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