Place the chicken breasts on a cutting board, cover with plastic wrap, and pound with a meat mallet or rolling pin to an even thickness; then remove the wrap and cut each breast in half so you have four sandwich-size pieces.
In a large bowl whisk together 1 cup buttermilk, 2 tablespoons hot sauce, 1 teaspoon paprika, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon ground black pepper, and 1 teaspoon kosher salt.
Add the chicken to the marinade, toss to coat, cover, and refrigerate for at least 1 hour or up to overnight.
In a shallow bowl combine 1 cup all-purpose flour, ½ cup cornstarch, 1 tablespoon paprika, 1 tablespoon garlic powder, 1 tablespoon onion powder, 1 teaspoon cayenne pepper, 1 teaspoon kosher salt, and 1 teaspoon ground black pepper.
Spoon about ¼ cup of the buttermilk marinade into the flour mixture and stir with a fork until small clumps form.
Dredge each marinated chicken piece in the flour mixture, pressing the coating onto the chicken to form a thick crust; then place the breaded pieces on a tray and refrigerate for 10 minutes.
While the chicken chills, heat the oil in a Dutch oven or heavy pot to 350°F, using a deep-fry thermometer to monitor the temperature.
Fry the chicken in batches until golden and cooked through, about 3–5 minutes per side depending on size; maintain oil temperature between batches and transfer cooked pieces to a paper towel–lined plate or wire rack to drain.
While the chicken cooks, make the spicy mayo by stirring ½ cup mayonnaise, 2 teaspoons hot sauce, 1 teaspoon honey, and ½ teaspoon garlic powder in a small bowl.
Melt 2 tablespoons unsalted butter in a skillet over medium-high heat and toast the brioche buns until golden.
To assemble each sandwich, spread spicy mayo on the bottom bun, top with 4 dill pickle slices, place the fried chicken, add a little more spicy mayo if desired, then top with the bun.