Line a 9-in. square pan with foil and grease the foil with butter; set aside. In a heavy saucepan, melt vanilla chips, 1/2 cup peanut butter and 1 tablespoon shortening over low heat; cook and stir constantly until smooth. Pour into prepared pan., In another heavy saucepan, melt milk chocolate chips, 1/2 cup peanut butter and 1 tablespoon shortening over low heat; cook and stir constantly until smooth. Drizzle over vanilla layer. Swirl with a knife. Refrigerate for 30 minutes or until firm. Using the foil, lift fudge out of pan. Gently peel off the foil; cut into 1-in. squares.