Preheat oven to 375 degrees F (190 degrees C).

Stir milk, egg, flour, garlic powder, salt, and black pepper together in a bowl.

Dredge each chicken breast in milk mixture; roll in cornflakes to coat. Transfer to a baking sheet or dish.

Bake until no longer pink at the bone and the juices run clear, about 45 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).