In a large bowl, whisk together the buttermilk, hot sauce, salt, pepper, garlic powder, and paprika.
Add the chicken thighs, making sure they are fully submerged in the marinade.
Cover the bowl and refrigerate for at least 4 hours, or preferably overnight.
In a large, shallow dish, whisk together the flour, cornstarch, cayenne pepper, paprika, garlic powder, onion powder, oregano, thyme, salt, and pepper.
Remove the chicken from the marinade, letting any excess drip off.
Dredge each piece of chicken thoroughly in the flour mixture, pressing firmly to ensure it's fully coated.
Place the dredged chicken on a wire rack while you heat the oil.
Pour enough frying oil into a large, heavy-bottomed pot or Dutch oven to reach a depth of about 2 inches.
Heat the oil over medium-high heat to 325-350°F (160-175°C). Use a thermometer to ensure the oil is at the right temperature.
Carefully add the chicken to the hot oil, being careful not to overcrowd the pot. Fry in batches if necessary.
Fry for 6-8 minutes per side, or until the chicken is golden brown and cooked through. The internal temperature should reach 165°F (74°C).
Remove the chicken from the oil and place it on a wire rack to drain.
Carefully reserve about 1/2 cup of the frying oil.
In a heatproof bowl, whisk together the reserved hot oil, cayenne pepper, paprika, brown sugar, and garlic powder. Adjust the amount of cayenne pepper to your desired level of heat.
Dip each piece of fried chicken into the hot oil, making sure it's fully coated.
Spread mayonnaise (if using) on the toasted brioche buns.
Place a piece of hot chicken on the bottom bun.
Top with dill pickle chips and coleslaw.
Place the top bun on the sandwich and serve immediately!