Heat oven to 400ºF. Spray cookie sheet with cooking spray.
Pour buttermilk into shallow glass or plastic bowl. Add chicken; turn to coat. Let stand 5 minutes.
Meanwhile, in 2-quart resealable food-storage plastic bag, crush cereal with rolling pin. Add Bisquick mix or flour and dressing mix (dry) to cereal in bag. Remove chicken from buttermilk; discard buttermilk. Add chicken to cereal mixture. Seal bag; shake to coat.
Place chicken on cookie sheet. Spray with cooking spray. Bake 45 to 50 minutes or until juice of chicken is clear when center of thickest part is cut (170°F).