Preheat the oven to 425 degrees. Line a baking sheet with parchment or foil.
Pour the buttermilk into a bowl or shallow dish. Whisk together the bread crumbs, panko, and ranch dressing mix in a bowl or shallow dish.
Dip each piece of chicken breast into the buttermilk, then roll in the bread crumb mixture to coat. Place on the prepared baking sheet and repeat with remaining chicken.
Spray the chicken with cooking spray and bake 20-25 minutes, or until cooked through. Serve hot with ranch dressing for dipping.