Buttermilk Fried Chicken

Name: Buttermilk Fried Chicken
Ingredients: 1 teaspoon brown mustard seeds; 1 teaspoon cumin seeds; 1 teaspoon fennel seeds; 5 ounces kosher salt (½ cup Diamond Crystal or ¼ cup Morton); ½ cup (packed) light brown sugar; 1 teaspoon black peppercorns; 2 bay leaves; 2 3½–4-pound chickens, cut into 8 pieces each; 6 cups all-purpose flour, divided; 1 tablespoon cornstarch; 1 tablespoon freshly ground black pepper; 1 tablespoon garlic powder; 1 tablespoon onion powder; 1 tablespoon smoked paprika; 1½ teaspoon ground fennel or finely ground fennel seeds; 1 tablespoon Diamond Crystal or 1½ teaspoons Morton kosher salt, plus more; 2 large eggs, beaten to blend; 2 cups buttermilk; Vegetable oil (for frying; about 6 cups); White Barbecue Sauce and bread-and-butter pickles (for serving); A deep-fry thermometer