01 - Slice each chicken breast in half horizontally to create 4 thin fillets of even thickness.
02 - Whisk buttermilk, garlic powder, onion powder, salt, and pepper in a bowl. Add chicken pieces and marinate for at least 15 minutes, up to 4 hours refrigerated for optimal flavor penetration.
03 - Prepare three shallow bowls: one with flour, one with beaten eggs, and one with panko breadcrumbs mixed with grated Parmesan cheese.
04 - Remove chicken from marinade. Dredge each piece in flour, shaking off excess. Dip into egg mixture, then coat thoroughly with panko-Parmesan mixture, pressing gently to adhere.
05 - Heat ½ inch of vegetable oil in a large skillet over medium-high heat. Fry chicken pieces for 3 to 4 minutes per side until golden brown and cooked through to internal temperature of 165°F. Transfer to paper towels to drain.
06 - Lightly toast brioche or sandwich buns until golden and crisp.
07 - Spread Caesar dressing on the bottom half of each bun. Layer with shredded romaine lettuce, a crispy chicken fillet, additional Caesar dressing, and shaved Parmesan. Finish with freshly ground black pepper.
08 - Place top bun on each sandwich and serve immediately while chicken remains crispy.