Pat the shrimp dry with a kitchen towel and season with 2 tablespoons of Cajun seasoning, garlic powder, and onion powder.
In a large bowl, combine the buttermilk, yellow mustard, and hot sauce. Mix well. Add the seasoned shrimp to the buttermilk marinade and marinate for at least 30 minutes.
In a separate bowl, mix the flour, cornstarch, and remaining tablespoon of Cajun seasoning until well combined.
Remove the shrimp from the buttermilk marinade and coat them evenly in the flour mixture. Repeat with the remaining shrimp.
Heat the oil in a wok or deep fryer over medium heat to 350°F. Once the oil reaches the correct temperature, carefully add the shrimp in batches and fry until golden and crispy.
Transfer the fried shrimp to a paper towel-lined plate to drain excess oil, then arrange them on a serving platter.
In a medium bowl, combine the mayonnaise, ketchup, lemon juice, rice vinegar, Dijon mustard, Worcestershire sauce, garlic, and paprika. Mix until smooth and well combined. Transfer the dipping sauce to a small serving bowl.
Garnish the Cajun fried shrimp with chopped parsley and serve with the dipping sauce and lemon wedges on the side.