Nashville Hot Chicken Sandwich

Name: Nashville Hot Chicken Sandwich
Ingredients: 1 1/2 cups (355ml) buttermilk; 3 tablespoons (45ml) Crystal hot sauce or other Louisiana-style hot sauce; 4 1/2 teaspoons kosher salt; for table salt use half as much by volume, divided; Six 3- to 5-ounce boneless skinless chicken thighs; 2 large large eggs; 1 1/4 cups all purpose flour (5 3/4 ounces, 150g); 1/4 cup cornstarch (1 1/3 ounces; 38g); 1 teaspoon cayenne pepper; 1 teaspoon mild sweet paprika; 1 teaspoon freshly ground black pepper; 1/4 teaspoon baking powder; 2 quarts (2L) neutral oil such as vegetable or peanut, for frying; 3 tablespoons (42g) unsalted butter; 1 to 3 tablespoons cayenne pepper (9 to 25g); 2 tablespoons packed dark brown sugar (about 1 ounce; 28g); 3/4 teaspoon onion powder; 3/4 teaspoon kosher salt; for table salt, use half as much by volume; 3/4 teaspoon mild sweet paprika; 1/4 teaspoon chile powder; 1/4 teaspoon freshly ground black pepper; 1/2 cup (118ml) reserved hot frying oil, from above; 6 brioche hamburger buns; 6 tablespoons (90g) mayonnaise, such as Duke’s brand; Dill or bread and butter pickle chips