Blend 1 cup yogurt and next 10 ingredients in processor until smooth.
Transfer to large bowl.
Mix in remaining 1 cup yogurt.
Add chicken and turn to coat. Cover and refrigerate overnight.
Prepare barbecue (medium-high heat).
Place marinade-coated chicken on barbecue. Grill chicken until just cooked through, occasionally basting with butter last 2 minutes, about 12 minutes per side.
Transfer chicken to platter. Top with onion and cilantro and serve.