In a large mixing bowl, whisk together the buttermilk, garlic powder, onion powder, smoked paprika, salt, and black pepper until well combined. Immerse the chicken breasts in the marinade, cover with plastic wrap, and refrigerate for a minimum of 2 hours, ideally overnight.
In a shallow dish or a large plate, combine the all-purpose flour, panko breadcrumbs, dried thyme, and cayenne pepper (if using). Mix well to ensure an even distribution of the spices.
Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Remove the chicken breasts from the buttermilk mixture, allowing excess to drip off. Dredge each piece in the flour mixture, ensuring even coating, and shake off any surplus.
Arrange the coated chicken breasts on the lined baking sheet, brush each piece with melted butter, and bake for 20-25 minutes until golden brown and fully cooked (internal temperature should be 165°F or 74°C).
Let the chicken rest on the baking sheet for about 5 minutes before serving. Sprinkle with freshly chopped parsley just before serving.
Notes
For an appealing presentation, serve the chicken on a large platter with parsley and lemon wedges.