Whisk the buttermilk, egg, and spices in a large bowl until combined. Add the chicken to the buttermilk mixture and place in the refrigerator, covered, for 30 minutes or up to 24 hours.
In a shallow bowl, whisk the dry ingredients; flour, cornstarch, and spices. Remove the chicken from the marinade and dredge in the flour mixture, making sure to coat well by pressing the flour on all sides of the chicken.
Deep fry the chicken in about 2 inches of oil for 4 to 5 minutes per side or until cooked through. Remove onto a paper towel.
While the chicken is frying, warm the butter, sweet chili sauce, and sriracha in a small pan just until the butter is melted.
Place the chicken in a deep bowl and add the sauce. Mix well just until the chicken is coated with the sauce.
Make the chili mayo topping, whisk the mayo, sweet chili sauce, and sriracha in a bowl. Toast the brioche buns in a tablespoon of butter.
Top each bun with a chicken fillet, sandwich toppings of choice (lettuce, onion, tomato, pickle), and about a tablespoon of the mayo mixture. Serve immediately and enjoy!
