*In large bowl, place chicken and buttermilk; toss to coat. Refrigerate at least 30 minutes, but no longer than 8 hours.
*Heat oven to 450°F. Place ovenproof rack on top of rimmed cookie sheet; spray both with cooking spray. Set aside.
*Place egg whites and bread crumbs in two separate shallow dishes or pie plates. Transfer chicken to large plate, letting buttermilk drip back into bowl. Discard buttermilk.
*Pat chicken dry with paper towels. Rub Cajun seasoning on both sides of chicken.
*Dip chicken into egg whites, letting excess drip back into dish. Coat chicken in bread crumbs, pressing to adhere. Lightly spray both sides of chicken pieces with cooking spray; place on rack.
*Bake 25 to 30 minutes, turning once halfway through cooking, until lightly browned and juice of chicken is clear when center of thickest part is cut (165°F). If desired, broil 6 inches from heating element 30 seconds to 1 minute to thoroughly brown and crisp the crust.