In a large bowl, mix buttermilk, garlic, jalapeño, shallot, 2 teaspoons kosher salt, and 1 teaspoon black pepper.
Pound the chicken breasts between the 2 pieces of parchment paper with a rolling pin until they are ½ inch thick. Place the chicken inmarinade, cover and refrigerate for at least 8 hours.
Preheat oven at 300F. Put a wire rack on a sheet pan in the oven.
In a bowl, flour, baking powder, cayenne pepper, paprika, celery salt, 1 tablespoon kosher salt, and 1½ teaspoons black pepper. Set aside.
Pour oil into a medium Dutch oven until it reaches 2 inches.Heat oil until it reaches the temperature of 350 degrees.
Take chicken pieces from marinade, dredge it in the flour mixture, dip again in the marinade and then in flour mixture. Shake off the excess and carefully transfer only 3 pieces into oil. Cook for 5 minutes on each side until brown. Transfer the chicken to the oven to keep them warm until ready to serve.
To assemble the chicken sandwich, place the bottom piece of bun on a serving plate, spread buttermilk herb mayo, place a leaf of lettuce, pickle slices, and chicken piece. Spread mayo on the top bun and place over the top of chicken. Serve immediately.