In a medium bowl, combine buttermilk, Sriracha, salt, pepper, and garlic powder. Add chicken thighs and marinate overnight, or at least for 4 hours.
Prepare the flour mixture by whisking together flour, cornstarch, paprika, salt, pepper, garlic powder, and cayenne powder in a shallow bowl.
Add 2 tbsp of the marinade into the mixture and mix through with a fork. Working with one piece at a time, dredge the chicken with the flour mixture, making sure it is completely coated.
In a small bowl, prepare the sauce by mixing together Sriracha and mayonnaise.
Heat the oil in a deep pan until it reaches 350˚F (180˚C). Fry the chicken for about 3-4 minutes on each side, or until cooked through and golden brown. The thickest part of the chicken should register 165˚F (75˚C).
Generously spread the Sriracha mayo on the toasted brioche buns, and top with sliced pickles, lettuce, and tomato. Serve immediately and enjoy!