Whisk the buttermilk with 1 teaspoon of cayenne pepper in a shallow bowl and soak the chicken for at least 15 minutes.
In a separate shallow dish, whisk together the flour, cornstarch, paprika, remaining cayenne, garlic powder, and onion powder.
Dredge each piece of chicken from the buttermilk into the flour mixture, pressing firmly to ensure a thick, even coating.
Heat the vegetable oil in a heavy-bottomed skillet or Dutch oven to 350 degrees Fahrenheit.
Fry the chicken for 5 to 7 minutes per side until the crust is deep golden brown and the internal temperature reaches 165 degrees Fahrenheit.
In a small bowl, whisk together the mayonnaise, sweet chili sauce, Sriracha, and honey to create the Bang Bang sauce.
Toast the brioche buns in a dry pan over medium heat until golden.
Assemble the sandwiches by spreading a generous amount of sauce on the top and bottom buns, then layering pickles, the fried chicken, and shredded lettuce.