Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
Combine bread crumbs, Parmesan cheese, yellow cornmeal, and Italian seasoning in a shallow dish.
Pour ranch dressing in a separate shallow bowl.
Dip chicken thighs and chicken breasts in ranch dressing to coat; press into the bread crumb mixture to coat both sides. Arrange chicken pieces in the prepared baking dish.
Cook chicken until no longer pink in the center and the juices run clear, 45 to 50 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
