Place chicken breasts between plastic wrap and pound to an even 1/2-inch thickness using a meat mallet or rolling pin.

Combine buttermilk and chicken in a shallow dish. Cover and refrigerate for at least 30 minutes, up to 4 hours for best results.

While the chicken marinates, make the bang bang sauce by whisking together mayonnaise, sweet chili sauce, sriracha, honey, minced garlic, and rice vinegar in a small bowl until smooth. Set aside.

In a shallow dish, whisk together flour, salt, black pepper, garlic powder, paprika, and cayenne pepper.

Remove chicken from buttermilk, letting excess drip off. Press each piece firmly into flour mixture to coat completely, then shake off excess flour.

Heat 3-4 cups of vegetable oil (about 2 inches deep) in a deep skillet or Dutch oven to 350°F (175°C). This will take 10-15 minutes. Use a thermometer to verify and maintain temperature between 325-375°F.

Carefully place chicken in hot oil and fry for 7 minutes per side until golden brown and internal temperature reaches 165°F. If your skillet is large enough, fry both pieces at once; otherwise, fry one at a time and keep the first piece warm in a 200°F oven. Transfer to a wire rack and let rest for 2-3 minutes.

Toast brioche bun halves lightly until golden. Spread bang bang sauce on both halves of each bun.

Assemble sandwiches by placing lettuce on bottom bun, adding fried chicken, topping with pickles and optional shredded cabbage, drizzling with additional sauce, and adding top bun. Serve immediately.