1. Line 8- or 9-inch square pan with wax paper.
2. Melt chocolate chips with sweetened condensed milk and salt in heavy saucepan over low heat, stirring constantly. Remove from heat; immediately stir in nuts, if desired, vanilla and 2/3 cup peanut butter chips. Spread evenly in prepared pan. Melt remaining 1 cup peanut butter chips with whipping cream over low heat, stirring until thick and smooth. Spread over top of chocolate.
3. Refrigerate 2 hours or until firm. Turn fudge onto cutting board; peel off paper and cut into squares. Store covered in refrigerator. About