In a shallow dish, whisk together the buttermilk, garlic powder, onion powder, salt, and pepper until well combined. Add the chicken breasts, making sure they are completely submerged in the marinade. Cover the dish with plastic wrap and refrigerate for a minimum of 30 minutes, up to 4 hours.
Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a separate bowl, combine the breadcrumbs with the grated Parmesan cheese. Remove the chicken breasts from the marinade, allowing any excess buttermilk to drip off. Dredge each breast in the breadcrumb mixture, pressing down firmly.
Heat a few tablespoons of olive oil in a large skillet over medium heat. Once the oil is shimmering, add the coated chicken breasts. Cook for about 5-7 minutes on each side, or until golden brown and cooked through. Alternatively, bake the breaded chicken on the prepared baking sheet for 20-25 minutes.
While the chicken is cooking, slice the sandwich buns in half and toast them lightly. Spread Caesar dressing on the cut sides of both halves. On the bottom half, layer the chopped romaine lettuce, followed by a crispy chicken breast. Drizzle more Caesar dressing over the chicken and finish with freshly shaved Parmesan.
Place the top half of the bun over the assembled ingredients. Serve immediately.
Notes
Serve the sandwiches on a rustic wooden board, garnished with extra romaine leaves and Parmesan shavings on the side.