Pat dry the shrimp with a kitchen towel and season them with 2 tablespoons of Cajun seasoning, garlic powder, and onion powder.
In a large bowl, combine buttermilk, yellow mustard, and hot sauce. Mix well. Add the seasoned shrimp to the buttermilk mixture and marinate for at least 30 minutes.
In a separate bowl, mix flour, cornstarch, and the remaining tablespoon of cajun seasoning until well combined.
Remove the shrimp from the buttermilk mixture and coat them evenly in the flour mixture. Repeat this step with the remaining shrimp.
Heat oil in a wok or deep fryer over medium heat (350 F). Once the oil is hot enough, carefully add the shrimp in batches and fry them in batches until they are golden and crispy.
Once done, drain the excess oil using a kitchen towel and transfer the shrimp to a serving platter.
Meanwhile, prepare the dipping sauce. In a medium bowl, combine mayo, 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 them with the dipping sauce and lemon wedges on the side.