Whisk together brown sugar, paprika, salt, and garlic powder in a small bowl. Use hands to rub the spice mix generously onto both sides of each chicken breast.
Preheat a large pan or nonstick skillet over medium heat. Spray with cooking spray. Add chicken breasts to pan and cook 4-5 minutes each side or until cooked through and juices run clear. Serve with steamed rice, mango slices, lime wedges and fresh cilantro if desired.