FOR THE CHICKEN:
FOR THE SANDWICHES:
Cut 2 boneless, skinless chicken breasts in half crosswise. Working with 1 piece at a time, place between 2 sheets of plastic wrap and gently pound with the flat side of a meat mallet until 1/2-inch thick. Transfer to a quart-size zip-top bag.
Add 1/2 cup garlic dill pickle brine (include some of the garlic and spices) to the bag. Flip the bag around to coat the chicken with the brine. Seal the bag and refrigerate for at least 4 or up to 8 hours.
Place 1/2 cup plus 2 tablespoons all-purpose flour, 1/3 cup panko breadcrumbs, 2 tablespoons Cajun seasoning, 2 tablespoons cornstarch, and 1/2 teaspoon MSG if using in a shallow bowl or pie plate and whisk to combine. Place 2 large egg whites and 1/2 cup buttermilk in a second shallow bowl and whisk to combine.
Working with 1 chicken cutlet at a time, remove from the marinade, dredge in the breadcrumb mixture until completely coated, then shake off all the excess. Dip in the egg white mixture until coated, then let the excess drip off. Return to the breadcrumb mixture and press gently so the mixture adheres to the chicken. Place on a plate. Let stand at room temperature for 15 minutes while you heat the air fryer and make the spicy mayo.
Heat the air fryer and air fryer basket to 390°F for 15 minutes. If you’d like to toast the buns, heat a regular or toaster oven to 350ºF. Place 1/2 cup mayonnaise, 1 tablespoon white miso, and 2 teaspoons hot sauce in a small bowl and whisk to combine.
Spray the top of the breaded chicken thoroughly with cooking spray. Working in 2 batches if needed to leave space between the pieces, place the chicken in the air fryer basket sprayed-side down. Spray the top of the chicken with more cooking spray.
Air fry for 6 minutes. Flip the chicken with a meat fork or two regular forks and air fry until the chicken is crisped, cooked through, and registers 165ºF on an instant-read thermometer, about 6 minutes more.
Split 4 brioche buns and place directly on the oven rack. Bake until lightly toasted, about 4 minutes. Meanwhile, cut 1 large dill pickle crosswise into 1/4-inch-thick rounds.
Place a piece of chicken on each bottom bun. Top each with a green leaf lettuce leaf and a few pickle slices. Spread the spicy mayonnaise on the cut side of the top buns, then close the sandwiches.