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.

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.

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 into the flour mixture to coat completely, then shake off excess flour.

Heat vegetable oil in a deep skillet or Dutch oven to 350°F (175°C).

Carefully place chicken in hot oil and fry for 7 minutes per side until golden brown and internal temperature reaches 165°F. 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.