Cut chicken into bite-sized pieces.
Place chicken in a bowl with buttermilk. Cover and refrigerate for at least 1 hour.
In a separate bowl, combine flour, salt, pepper, garlic powder, onion powder, paprika, and cayenne pepper.
Remove chicken from buttermilk, letting the excess drip off, and coat in the flour mixture.
Heat oil in a deep skillet or fryer to 350°F (175°C).
Fry the chicken in batches until golden brown and cooked through, about 4–5 minutes.
Drain on paper towels.
Serve warm with Thai sweet chili sauce, garnished with green onions and sesame seeds if desired.