Place the cleaned shrimp in a medium bowl. Add buttermilk and 1 tablespoon hot sauce, stir to coat. Refrigerate and let marinate 15–30 minutes. This tenderizes the shrimp and helps the coating adhere.

Prep dry mix

While shrimp marinate, whisk together the all-purpose flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, 1 teaspoon salt, and 1/2 teaspoon black pepper in a shallow bowl or rimmed plate.

Heat oil

Pour vegetable oil into a heavy skillet or Dutch oven to a depth of about 1 to 1.5 inches (roughly 3–4 cups depending on pan). Heat over medium-high until the oil reaches 350°F (175°C) or a wooden spoon handle inserted will form steady bubbles.

Dredge and fry

Remove shrimp from buttermilk, letting excess drip off. Dredge each shrimp in the flour mixture, pressing lightly to adhere. Shake off excess. Fry shrimp in batches (do not overcrowd) 2–3 minutes per side until golden brown and cooked through. Use a slotted spoon to transfer to a wire rack or paper towel-lined tray.

Season after frying

Immediately after each batch, brush or drizzle the shrimp with a little melted butter and sprinkle with 1 teaspoon Cajun/Creole seasoning while still hot so the seasoning sticks. Repeat until all shrimp are cooked.

For the remoulade

In a small bowl, combine 1/2 cup mayonnaise, 1 tablespoon Creole or Dijon mustard, 1 teaspoon prepared horseradish, 1 teaspoon hot sauce, 1 tablespoon lemon juice, 1/2 teaspoon Worcestershire sauce, 1/2 teaspoon paprika, 1/4 teaspoon smoked paprika (if using), 1/4 teaspoon salt, 1/4 teaspoon black pepper, 2 teaspoons finely chopped celery, and 1 tablespoon finely chopped green onion. Taste and adjust acidity or heat. Refrigerate until assembly (can be made ahead up to 2 days).

For the sandwich and assembly

Prepare bread and toppings

If desired, lightly toast the split French bread or hoagie rolls under a broiler or in a hot skillet for 1–2 minutes until slightly crisp. Spread softened butter on cut sides if using. Lay out shredded lettuce, tomato slices, pickle slices, and optional banana pepper rounds.

Build the Po' Boy

Spread a generous layer (about 1 to 1.5 tablespoons) of remoulade on both sides of each roll. Pile a good handful of shredded lettuce along the bottom half. Arrange hot fried shrimp evenly along the roll (approximately 4–6 shrimp per sandwich depending on size). Top with tomato slices and pickle slices. Add extra remoulade on top if desired.

Serve

Close sandwich, press gently, and serve immediately while shrimp are hot and crisp. Provide extra hot sauce or lemon wedges on the side.