In a large bowl, combine the chicken with buttermilk, salt, pepper, and cayenne powder. Cover with plastic wrap and marinate in the fridge for at least 30 minutes and up to 24 hours.
For the bread, in a shallow bowl, combine flour with cornstarch, paprika, cayenne powder, garlic powder, salt, and pepper.
For extra flaky chicken sandwich, add a tablespoon or so of the buttermilk mixture to the flour mixture, and mix with your hands to create little clumps.
Dredge the chicken in the flour mixture. Press the chicken into the flour mixture to make sure it sticks to the chicken.
Lightly shake off any excess flour, and place on a plate but do not pile the chicken.
Heat vegetable (or canola) oil to 350°F (180°C) in a Dutch oven or a deep fryer. Fry the chicken for 3-4 minutes per side, the chicken is ready when the internal temperature reaches 165°F (74°C).
To make the spicy mayo, in a small bowl whisk mayonnaise with hot sauce and paprika, and set aside.
To toast the brioche buns, slice them in half then melt 1 tablespoon butter to a skillet on medium-high heat, and place one bun cut in half cut-side down. Toast until they are golden brown but not burnt. Repeat with the second bun.
To assemble the chicken sandwich, spread 1 tablespoon of spicy mayo on each half bun. On the bottom bun layer dill pickle slices over the sauce, place the chicken on top of the pickles, and cover with the top half bun.