Marinate the chicken by mixing buttermilk, salt, and cayenne pepper in a bowl. Add the chicken breasts and refrigerate for at least 1 hour or overnight.
Prepare the ranch slaw by whisking together mayonnaise, sour cream, lemon juice, apple cider vinegar, parsley, dill, salt, and pepper. Toss in the shredded cabbage and julienned carrots, then chill the mixture.
Create the dredging mixture by combining flour, cornstarch, paprika, garlic powder, onion powder, salt, black pepper, and cayenne pepper in a bowl. Dredge the marinated chicken in this mixture.
Fry the chicken by heating oil to 350°F (175°C) and frying the coated chicken until golden brown and the internal temperature reaches 165°F. Once cooked, let the chicken rest on a wire rack.
Sauce the chicken by tossing or brushing the hot fried chicken with buffalo sauce.
Assemble the sandwich by toasting the brioche buns, adding the buffalo chicken, topping with ranch slaw, and serving immediately.
