Preheat oven to 375°.
Remove and discard giblets and neck from chicken; trim excess fat. Starting at neck cavity, loosen skin from breasts and drumsticks by inserting fingers, gently pushing between skin and meat.
Combine butter, chile powder, rind, 1/2 teaspoon salt, cumin, and 1 garlic clove in a small bowl. Rub seasoning mixture under loosened skin and over breasts and drumsticks. Tie ends of legs together with twine. Lift wing tips up and over back; tuck under chicken. Place chicken, breast side up, on a rack coated with cooking spray; place rack in roasting pan.
Discard husks and stems from tomatillos; cut into quarters. Arrange tomatillos and onion evenly around chicken. Bake at 375° for 40 minutes.
Increase oven temperature to 450°, and bake an additional 20 minutes or until a thermometer inserted in the meaty part of thigh registers 170°. Remove chicken from pan; let stand 15 minutes.
Place tomatillos and onion in a food processor. Add remaining 1/4 teaspoon salt, remaining 3 garlic cloves, cilantro, juice, and chiles; process until smooth.
Remove skin from chicken; discard. Carve chicken, and serve with sauce. Sprinkle with cilantro, if desired.