In a large bowl, whisk together the milk and pickle juice.
Add the chicken to the milk mixture and toss to coat. Cover with plastic wrap and refrigerate for at least 8 hours or up to (but not more than) 24 hours.
When you’re ready to cook the chicken, preheat the oven to 200° F. Set a wire rack on a sheet pan and set aside.
In a large bowl, whisk together the flour, confectioners’ sugar, baking powder, paprika, salt, pepper, garlic powder and onion powder.
Pour oil to a depth of 1 ½ inches in a Dutch oven. Heat the oil over high heat until it reaches 350° F.
While the oil heats, remove the chicken from the milk mixture. Add an egg to the milk mixture and whisk well to combine. Dip the chicken in the milk/egg mixture, dredge the chicken in the flour mixture, submerge it again in the milk/egg mixture, and then dredge again in the flour mixture. Shake off any excess egg or flour after each dip. Transfer the chicken to a plate or sheet pan until ready to fry.
Once the oil comes to temperature, use tongs to carefully place two chicken breasts in the oil. Adjust the heat to maintain a temperature of 350° F. Cook for a total of 5 minutes, flipping once halfway through. Transfer the fried chicken to the wire rack on the baking sheet. Season with salt and place in the oven to keep warm. Repeat with the remaining two chicken breasts.
Assemble the sandwiches on the griddled buns with optional toppings. Serve immediately.