Place each chicken breast between two sheets of parchment paper or plastic wrap and pound with a meat mallet or rolling pin until 1/2 inch thick; slice each breast in half to make six pieces.
Whisk together the buttermilk, hot sauce, and dill pickle juice in a large bowl; add the chicken pieces, submerge, and refrigerate to soak while you prepare the coating.
In a shallow bowl, whisk the flour with the sea salt, black pepper, paprika, and onion powder until evenly combined.
Pour the oil into a Dutch oven or deep heavy pot to a depth of at least 4 inches and heat to 350°F (use a candy or instant-read thermometer to check).
Remove a chicken piece from the buttermilk, dredge in the flour mixture to coat both sides, dip back into the buttermilk, then dredge in flour again to create two coats.
Carefully lower one piece at a time into the hot oil and fry, turning once, until deep golden brown and the internal temperature reaches 165°F, about 4–6 minutes per piece; maintain the oil at 350°F.
Transfer cooked chicken to a paper towel–lined tray to drain excess oil.
Split the buns and lay cut-side up on a baking sheet; spread each with softened butter and broil 2–3 minutes until just toasty.
Assemble sandwiches by placing several dill pickle slices on the bottom bun, topping with a fried chicken piece, spreading on the Copycat Chick-fil-A sauce, and finishing with the top bun; serve immediately.
