Prepare the chicken: Place chicken cutlets in a shallow dish. Pour buttermilk over the chicken, add salt and pepper, and let it marinate for at least 30 minutes or up to 4 hours in the refrigerator.
Prepare the dredge: In a separate shallow dish, whisk together the flour, cornstarch, paprika, and garlic powder.
Coat the chicken: Remove one cutlet from the buttermilk, letting excess drip off. Dredge it thoroughly in the flour mixture, pressing firmly to coat. Dip it back into the buttermilk quickly, then dredge it again in the flour mixture, ensuring a thick, craggy coating. Place coated chicken on a wire rack. Repeat for all cutlets.
Make the slaw: In a medium bowl, whisk together the mayonnaise, apple cider vinegar, sugar, Dijon mustard, salt, and pepper until smooth. Add the shredded cabbage and carrots. Toss well to coat. Refrigerate for at least 20 minutes before serving.
Fry the chicken: Heat oil in a deep skillet or Dutch oven to 350°F (175°C). Carefully place two chicken cutlets into the hot oil, ensuring not to overcrowd the pan. Fry for 4 to 6 minutes per side, until golden brown and the internal temperature reaches 165°F (74°C).
Drain the chicken: Remove the cooked chicken and place it on a clean wire rack set over a baking sheet. Sprinkle lightly with salt immediately. Keep warm while you fry the remaining pieces.
Toast the buns: Lightly butter the cut sides of the brioche buns. Toast them in a dry skillet over medium heat until golden brown.
Assemble the sandwich: Spread mayonnaise on both halves of each toasted bun. Place one crispy chicken fillet on the bottom bun. Top the chicken generously with the creamy coleslaw. Place the top bun on the slaw. Serve immediately.
Notes
For extra crispiness, you can use an air fryer after dredging: Spray the coated chicken lightly with cooking spray and air fry at 380°F (195°C) for 12-15 minutes, flipping halfway.
Add a dash of hot sauce to the buttermilk marinade for a spicy chicken sandwich variation.
Use potato rolls instead of brioche for a softer bun texture.