Whisk together buttermilk, hot sauce, salt, and black pepper in a medium bowl. Add chicken breasts and make sure they are fully submerged. Cover and refrigerate for at least 30 minutes or up to 4 hours. Note: marinating time is not included in the prep time listed.

In a shallow dish, whisk together flour, cornstarch, garlic powder, onion powder, paprika, and cayenne if using until evenly combined.

Remove chicken from the buttermilk and let excess drip off. Press each piece firmly into the flour mixture to coat all sides completely. Set on a wire rack and rest for 5 minutes to help the coating adhere.

Pour about 1 inch of vegetable oil into a heavy-bottomed skillet or Dutch oven. Heat over medium-high until it reaches 350 degrees F. Test by dropping a small pinch of flour coating into the oil. If it bubbles and turns golden within about 5 seconds, the oil is ready.

Fry chicken in batches without crowding the pan, about 6 to 8 minutes per side, until deeply golden brown and the internal temperature reads 165 degrees F at the thickest part.

Transfer cooked chicken to a wire rack to drain. Do not set directly on paper towels as this traps steam and softens the crust.

Whisk together mayonnaise, grated Parmesan, lemon juice, Dijon mustard, minced garlic, Worcestershire sauce, and black pepper. Slowly drizzle in olive oil while whisking constantly until smooth and slightly thickened. Taste and adjust seasoning.

Lightly toast brioche buns if desired. Spread Caesar dressing on both cut sides. Layer romaine on the bottom bun, place crispy chicken on top, add shaved Parmesan and tomato slices if using. Top with the upper bun and serve immediately.