In blender, place peanut butter, yogurt and sugar. Cover; blend on high speed 45 to 60 seconds until well mixed. Pour mixture into chilled bowl of ice-cream freezer.
Churn until thickened, 20 to 25 minutes, adding chocolate chunks during the last 2 minutes. Frozen yogurt will be soft-serve and ready to eat, or transfer to freezer container and freeze until firm enough to scoop, about 4 to 6 hours.