Step 1: Marinate the Chicken

1. In a bowl, mix buttermilk, salt, black pepper, smoked paprika, garlic powder, and onion powder.
2. Add the chicken breasts, making sure they are fully submerged. Cover and refrigerate for at least 30 minutes (or up to 8 hours for extra tenderness).

Step 2: Prepare the Breading

1. In one shallow bowl, mix flour, panko, baking powder, cayenne (if using), and a pinch of salt and pepper.
2. In another bowl, whisk together the egg and milk.

Step 3: Coat and Fry the Chicken

1. Remove chicken from the buttermilk marinade and let excess drip off.
2. Dip each piece into the egg mixture, then coat with the flour mixture, pressing lightly to adhere.
3. Heat about ½ inch of vegetable oil in a skillet over medium heat until it reaches 350°F (175°C).
4. Fry the chicken for 3-4 minutes per side, until golden brown and crispy. Transfer to a wire rack to drain.

Step 4: Make the Romesco Sauce

1. In a food processor, combine roasted red pepper, toasted almonds, olive oil, garlic, red wine vinegar, smoked paprika, salt, black pepper, and red pepper flakes.
2. Blend until smooth. Taste and adjust seasoning as needed.

Step 5: Assemble the Sandwich

1. Toast the focaccia slices lightly for extra texture.
2. Spread a generous layer of romesco sauce on both sides.
3. Add a crispy chicken piece, followed by arugula, sliced red onions, and tomatoes.
4. Close the sandwich and serve immediately.