In a medium bowl, combine buttermilk, Sriracha, salt, pepper, and garlic powder. Add chicken thighs and marinate in the fridge for at least 4 hours or overnight.
Whisk together flour, cornstarch, paprika, salt, pepper, garlic powder, and cayenne powder in a large bowl.
Add 2 tbsp of the marinade to the mixture and mix with a fork. Dredge the chicken with the flour mixture, making sure each piece is evenly coated. Prepare the sauce by mixing together Sriracha and mayonnaise.
Heat oil in a deep skillet until it reaches 350˚F (180˚C). Fry the chicken for about 3-4 minutes on each side, or until golden brown. The thickest part of the chicken should register 165˚F (75˚C).
Generously spread the Sriracha mayo on both sides of the buns and add your preferred toppings and the chicken. Serve immediately and enjoy!