Place chicken breasts between 2 large sheets of plastic wrap. Pound thickest side with the flat side of a meat mallet just until chicken is uniform in thickness, not too thin.
Place chicken into a large plastic storage bag or a shallow storage container. Whisk together buttermilk and hot sauce, pour over chicken. Seal bag and chill for 4 hours or overnight to marinate.
To fry: In a large dutch oven heat oil to 350°F. Have nearby a baking pan fitted with an oven safe rack.
Remove chicken pieces from marinade, allowing excess to drip off. Discard marinade. Season the chicken pieces lightly with salt. (eyeball amount, keep it light)
In a shallow dish whisk together flour, corn starch and baking powder. Add seasoned salt, pepper, onion powder, granulated garlic, mustard, paprika, dill, cumin and cayenne, if using. Whisk until seasonings are evenly distributed.
In a separate shallow bowl whisk together eggs with cold water.
To dredge: Dip chicken pieces in flour mixture, then into egg and back into flour mixture. Press coating firmly onto the chicken pieces. (See Cook's note for double dipping) Fry pieces in batches for 6 minutes per side or until cooked through. Adjust based on weight and size of chicken pieces used. Carefully remove to prepared pan. Season lightly with additional salt and keep warm.
Serve on toasted brioche buns with pickles, mayonnaise, lettuce or any of your favorite toppings.