In a medium bowl, whisk together buttermilk, egg, hot sauce, and kosher salt until well combined. Add the boneless, skinless chicken breasts, ensuring each piece is coated in the marinade. Cover the bowl and let it marinate in the refrigerator for at least 15 minutes.

In a separate large bowl, mix together all-purpose flour, cornstarch, seasoned salt, mustard powder (if using), ground black pepper, and ground thyme (if using). Stir until fully combined.

In a deep fryer or large pot, pour enough vegetable oil to cover the chicken pieces by at least an inch. Heat the oil to 350°F (175°C).

Remove the chicken from the marinade, allowing excess to drip off. Dredge each piece in the dry mixture, pressing gently for a thorough coating.

Carefully drop the coated chicken pieces into the hot oil. Fry until golden brown, about 3-5 minutes per batch. Use a slotted spoon to remove and drain on paper towels.

Arrange the fried popcorn chicken on a serving platter and enjoy with a variety of dipping sauces.