Mix first 4 ingredients in heavy saucepan. Bring to boil over medium heat, stirring constantly.
Dip 1 cup out in a bowl and add chocolate and vanilla. Stir till chocolate melts.
Pour chocolate filling in bottom of pie shell. Chill in refrigerator.
Add gelatin to 1/4 cup cold water. Let sit 1 minute.
Add gelatin water to hot custard. Stir till dissolved.
Stir in 1/2 cup peanut butter till well mixed and smooth.
Beat egg whites with electric mixer, gradually adding 1/4 cup sugar. Mix till stiff peaks form.
Fold in peanut butter custard and Cool Whip.
Pour peanut butter layer on top of chocolate layer. Chill in refrigerator.
Garnish with chocolate curls, if desired.