1.For marinade, in a blender combine mint leaves, lemon juice, oil, soy sauce, chili powder, pepper, and garlic. Cover and blend until smooth.
2.Butterfly chicken and cut in halve. Place chicken in a self-sealing plastic bag set in a shallow dish. Pour marinade over chicken. Seal bag; turn to coat chicken. Marinate in refrigerator for 4 to 24 hours, turning bag occasionally.
3.Drain chicken, discarding marinade. For a charcoal grill, place chicken on the rack of an uncovered grill directly over medium coals. Grill for 12 to 15 minutes or until no longer pink (170°F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place chicken on grill rack over heat. Cover and grill as above.)