Line a 13-in. x 9-in. pan with foil and grease the foil with butter; set aside. In a large heavy saucepan, melt the milk chocolate chips, 1/2 cup peanut butter and 1 tablespoon shortening over low heat; cook and stir constantly until smooth. Pour into prepared pan. Refrigerate for 10 minutes or until firm.
Meanwhile, in a microwave, melt the vanilla chips, the remaining peanut butter and shortening at 70% power for 1 minute; stir. Microwave at additional 10- to 20-second intervals, stirring until smooth.
Spread evenly over the chocolate layer. 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.Yield: About 2-1/2 pounds.