1. Line 8-inch square pan with foil.
2. Combine chocolate chips, sweetened condensed milk, vanilla and salt in heavy saucepan. Cook over low heat, stirring constantly, until chips are melted and mixture is smooth. Remove from heat. Add peanut butter chips; stir quickly just to distribute chips throughout mixture. Spread evenly in prepared pan.
3. Refrigerate 2 hours or until firm. Remove from pan; peel off foil. Cut into squares. Store tightly covered in refrigerator. 3 dozen pieces or about 2 pounds.
NOTE: For best results, do not double this recipe.