Lightly coat an unheated panini griddle, covered indoor electric grill, or large nonstick skillet with nonstick cooking spray. Heat according to manufacturer's directions or preheat over medium heat. Add chicken. If using griddle or grill, close lid and grill for 6 to 7 minutes or until chicken is no longer pink. (If using skillet, cook chicken for 10 to 12 minutes or until no longer pink, turning once.) Cool chicken slightly; split each chicken piece in half horizontally and cut crosswise into 2-inch-wide slices.