1. Prepare 9-inch baked pie shell as directed; cool.
2. Meanwhile, in heavy 2-quart saucepan, with wire whisk, beat milk, cornstarch, egg yolks, and 1/3 cup sugar until blended. Sprinkle gelatin evenly over mixture; let stand 1 minute to soften gelatin slightly. Cook over low heat, stirring constantly, until mixture thickens and boils; boil 1 minute. Spoon 1 cup mixture into small bowl; stir in peanut butter until blended. Stir melted chocolate into remaining mixture in saucepan. Refrigerate both mixtures until each mounds slightly when dropped from a spoon; then spoon chocolate mixture into cooled pie shell.
3. In large bowl, with mixer at high speed, beat egg whites until soft peaks form; gradually sprinkle in 1/3 cup sugar, 2 tablespoons at a time, beating until sugar completely dissolves and whites stand in stiff peaks.
4. In small bowl, with same beaters and with mixer at medium speed, beat 1/4 cup heavy cream until stiff peaks form. With rubber spatula or wire whisk, fold peanut-butter mixture and whipped cream into beaten egg whites; spoon mixture evenly over chocolate mixture in pie shell. Refrigerate pie 2 hours or until filling is set.