Preheat oven to 425°F. Line a sheet pan with greased foil.
If chicken breasts are large, cut in half. Cover chicken breasts with a layer of plastic wrap and pound to about 1 inch thick.
Sprinkle chicken generously with salt and pepper.
Add crushed cornflakes to a wide, shallow bowl. In a separate bowl, whisk together egg and milk.
Dip chicken breasts in egg mixture, then transfer to cornflake crumbs, making sure chicken is completely coated on all sides with the crumbs.
Place chicken on the prepared sheet pan. Drizzle with melted butter, trying to coat as much of the crumb coating as possible. Finish with a light spritz of the nonstick cooking spray to make sure all of the crumbs are moistened.
Bake until a thermometer inserted into the thickest part of the chicken breasts reads 165°F, about 20-25 minutes.