Prepare the chicken: Pound the chicken halves to an even 1/2-inch thickness.
Set up the breading station: In one shallow dish, mix the flour, salt, pepper, paprika, and garlic powder. In a second dish, place the buttermilk. In a third dish, place the beaten eggs.
Dredge the chicken: Dip each piece first into the flour mixture, shaking off excess. Next, dip into the buttermilk, letting excess drip off. Finally, coat thoroughly in the egg, then return to the flour mixture, pressing firmly to create a thick, craggy coating. Set aside on a rack for 10 minutes.
Make the sauce: Whisk together the mayonnaise, Dijon mustard, honey, apple cider vinegar, and salt in a small bowl until smooth. Refrigerate until ready to use.
Fry the chicken: Pour about 1 inch of vegetable oil into a deep skillet and heat to 350°F (175°C). Carefully place the coated chicken 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 wire rack set over a baking sheet to drain excess oil.
Toast the buns: Lightly toast the cut sides of the brioche buns in a dry pan or under the broiler until golden.
Assemble the burger: Spread a generous amount of honey mustard sauce on both the top and bottom buns. Place lettuce and pickles on the bottom bun, top with a crispy chicken fillet, and finish with the top bun. Serve immediately.
Notes
For extra crispiness, you can add 1 tablespoon of cornstarch to the initial flour mixture.
If you prefer baking, spray the breaded chicken with cooking spray and bake at 400°F (200°C) for 20-25 minutes, flipping halfway.
Brioche buns hold up well to the sauce and crispy texture.