Remove fat from chicken; make 3 or 4 long slits in each piece to allow marinade to penetrate.
Mix together yogurt, 1/4 cup chopped mint or cilantro, ginger, coriander, cayenne, salt and sugar. Set aside half the mixture. Add chicken to the remaining yogurt mixture; marinate 10 minutes, turning once.
Melt butter over medium-high heat in a nonstick skillet just large enough to hold the chicken. Remove chicken from marinade; discard marinade. Add onion, garlic and chicken to skillet. Brown chicken 3 minutes. Turn and brown 2 minutes. Lower heat to medium.
Spoon reserved yogurt mixture over the chicken. Cover and cook for 5 minutes or until cooked through. Sprinkle with the remaining 2 tablespoons chopped mint. Serve with jasmine rice.