Slice chicken breasts into sandwich-sized fillets and marinate in buttermilk, hot sauce, paprika, garlic powder, onion powder, Cajun seasoning, salt, and pepper for at least 1 hour.
Combine flour, corn starch, paprika, garlic powder, onion powder, Cajun seasoning, salt, and pepper in a shallow dish.
Heat oil in a skillet or fryer to 350°F (175°C).
Coat each chicken fillet in the dry batter mix.
Fry chicken fillets for 4–6 minutes per side until golden brown and cooked through.
Mix mayo, hot sauce, and Cajun seasoning for the sandwich spread.
Toast brioche buns, spread spicy mayo on both halves, add pickles, and top with fried chicken.