Fried Chicken Sandwich

Name: Fried Chicken Sandwich
Ingredients: 3/4 cup buttermilk, divided; 1/4 cup pickle brine; 1 tablespoon hot sauce, such as Crystal or Tabasco; 3 1/2 teaspoons kosher salt, divided; 2 1/2 teaspoons ground black pepper, divided; 2 teaspoons garlic powder, divided; 6 boneless, skinless chicken thighs (1 1/2 to 2 pounds total); 1 1/2 cups all-purpose flour; 2 tablespoons cornstarch; 1 teaspoon cayenne pepper; 3/4 teaspoon paprika; 2 large eggs; 4 cups vegetable oil, such as canola; 18 to 24 dill bread-and-butter pickle slices; 6 brioche buns; 1/2 cup mayonnaise