Combine chicken, buttermilk, egg, and 1 tablespoon seasoning mix in a plastic bag or bowl. Marinate in the fridge for 4 to 6 hours.
In a shallow bowl, mix flour, cornstarch, baking powder, salt, and the remaining seasoning mix to create the batter.
Mix shredded cabbage, carrots, mayonnaise, white vinegar, sugar, and salt to make the tangy slaw.
Heat peanut oil in a deep pan until a drop of water sizzles. Remove excess marinade from chicken, coat pieces evenly in the dry flour mix, and fry for 5 minutes on one side, then 2-3 minutes on the other side until golden and crispy. Drain on paper towels and sprinkle with salt.
Assemble sandwiches with brioche buns, fried chicken, Swiss cheese, bacon, pickles, honey dijonnaise, and tangy slaw.
Notes
For extra flavor, use homemade seasoning mix and let chicken marinate overnight. Serve immediately for the best crunch.