In a large bowl, combine buttermilk, fish sauce, red curry paste, garlic powder, and onion powder, mixing until smooth. Add the chicken, cover, and refrigerate for at least 2 hours, or preferably overnight.

Set up a dredging station by placing flour, cornstarch, salt, and black pepper in a shallow dish, whisking to combine evenly.

In a deep frying pan, pour vegetable oil to a depth of about 2 inches and heat it to 350°F (175°C).

Remove chicken from the marinade, allowing excess liquid to drip off. Dredge in the flour mixture, ensuring an even coating.

Carefully place chicken pieces into the hot oil, frying for about 6-8 minutes per side or until golden brown and internal temperature reaches 165°F (74°C).

In a small bowl, whisk together mayonnaise, sriracha, and lime juice until smooth. Adjust sriracha to your desired spiciness.

Lightly toast the brioche buns on a skillet or toaster until golden.

Spread the spicy mayo on both halves of the buns, layer with fried chicken, shredded cabbage, pickled carrots, and fresh cilantro. Top with the other half of the bun.

Serve immediately for the best flavor and texture.