Pound out your chicken breasts lightly and trim any fat off them if needed.
In a small bowl combine the cajun seasoning, pepper, garlic powder, onion powder and sugar.
To your 1 cup of buttermilk add 1 tablespoon of the seasoning mix.
Pour the buttermilk mix over the chicken.
Let the chicken marinate 1 hour up to overnight.
When you are ready to fry combine the remaining seasoning mix with the flour and cornstarch and whisk to combine.
Take 3-4 tablespoons of the buttermilk marinade and stir it into the flour mix, you want to make chunky pieces of flour to give the chicken some good crispy bits.
Dredge the chicken in the flour mix and dip back into the buttermilk and dredge in the flour a second time. Really try to press any of the chunky flour pieces into the chicken.
In a large heavy pot or deep frying pan add 4 cups of oil or enough to cover the chicken ¾ of the way.
Heat the oil over medium high heat until a little flour dropped in the pan sizzles immediately.
Add 2 pieces of chicken to your oil and fry 5-6 minutes, flip and repeat with the other side. Thicker breasts might need more time and thinner might need less, use a meat thermometer to check and make sure your chicken is at 165 degrees.
Remove you chicken breasts to a cooling rack and repeat with the remaining chicken breasts.
Time to assemble the sandwiches.
You can put them right on the brioche buns and serve.
You can take it a step further and butter the buns and toast them in a pan.
If you like spread the buns with mayo and add some thick pickle slices.
Serve!