Pour 1 cup yogurt in a large bowl, add salt, pepper and chili powder; whisk to mix well. Add the chicken pieces, turning to coat well. Cover with plastic wrap; refrigerate and marinate chicken for about 3 hours.
Preheat oven to 425 deg. Remove the chicken pieces from the bowl and gently shake off excess yogurt; discard marinade. Place the chicken in a 13x9-in baking dish and bake for about 40 minutes or until chicken is browned and juices run clear when pierced with a fork. While the chicken is cooking, prepare the sauce.
Heat the oil in a nonstick saucepan over medium heat and saute the onion until soft and beginning to brown. Add the garlic and chili powder; continue to cook for about 3 minutes, stirring constantly. Transfer the onion mixture to a blender or food processor bowl. Add the cilantro leaves, yogurt, chilies and salt. Stir in the sour cream. Season to taste with pepper. Pulse until the cilantro is finely chopped.
Coat a serving platter with the yogurt-cilantro sauce, place the chicken pieces on top and garnish with cilantro sprigs. Serve additional sauce on the side.