For the marinade: Combine the ketchup, sugar, vinegar, Worcestershire, cumin, paprika, garlic, 1/2 teaspoon salt and a few turns of pepper in a large resealable plastic bag. Add 4 boneless skinless chicken breasts or 8 boneless skinless thighs (about 1 1/2 pounds), squeeze out the air, seal tightly and massage gently to coat the chicken with marinade. Refrigerate for 2 hours or freeze up to 1 month; defrost in the refrigerator overnight before cooking.
To cook: Remove the chicken from the bag and grill over indirect heat, covered, until cooked through, lightly charred and an instant-read thermometer registers 165 degrees F, 20 to 25 minutes.
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