In a large bowl, whisk together the buttermilk, egg, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp cayenne, 1 tsp dried parsley, 1 tsp salt, and 1 tsp pepper. Add the halved chicken breasts, cover, and refrigerate for at least 30 minutes or up to 24 hours.
In a shallow bowl, combine the flour, cornstarch, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp paprika, 1 tsp cayenne, and 1 tsp salt. Mix well.
Remove each chicken piece from the marinade, letting excess drip off, then press and dredge thoroughly in the flour mixture so each piece is evenly coated.
Heat about 2 inches of neutral oil in a large heavy skillet or deep fryer to 350°F (175°C). Fry the chicken 4–5 minutes per side, until golden brown and cooked through (internal temperature 165°F/74°C). Transfer to paper towels to drain.
While frying, warm 4 tablespoons butter, 4 tablespoons sweet chili sauce, and 1 tablespoon sriracha in a small saucepan over low heat until the butter melts and the sauce is combined.
Place the fried chicken in a large bowl, pour the warm sticky chili glaze over it, and toss gently until each piece is evenly coated.
Make the chili mayo by whisking together 1/2 cup mayonnaise, 2 tablespoons sweet chili sauce, and 1 tablespoon sriracha in a small bowl.
Toast the brioche buns in 1 tablespoon butter in a skillet until golden. Assemble sandwiches with a glazed chicken fillet on each bun, add desired toppings (lettuce, onion, tomato, pickles), and spread about 1 tablespoon chili mayo on each sandwich. Serve immediately.
