In a large mixing bowl, combine the buttermilk and chicken breasts. Ensure the chicken is fully submerged in the buttermilk. Allow it to marinate in the refrigerator for at least 1 hour, or ideally overnight.
In a separate bowl, whisk together the flour, paprika, cayenne pepper, garlic powder, onion powder, and salt. This will be the coating for the chicken.
Heat vegetable oil in a large skillet or deep-fryer to 350°F (175°C). Make sure the chicken fully submerge in oil.
Take out the chicken from the buttermilk, and let any excess drip off. Dip each chicken breast into the flour mixture, ensuring it is fully coated. Shake off any excess flour.
Carefully place the chicken breasts into the hot oil, using tongs. Fry for about 6-8 minutes per side or until golden brown and cooked through. Use a deep-fry thermometer to ensure the chicken reaches an internal temperature of 165°F (74°C).
Do not waste time and prepare hot sauce until the chicken is frying. In a small bowl, whisk together the hot sauce, melted butter, brown sugar, cayenne pepper, smoked paprika, garlic powder, and onion powder.
Once the chicken is cooked, transfer it to a cooling rack lined with paper towels to drain excess oil. Immediately brush the hot sauce mixture over the chicken, ensuring it is evenly coated.
Optional: Lightly toast the brioche buns on a toaster or grill pan.
To assemble the sandwich, place a hot chicken breast on the bottom half of each bun. Top it with pickle slices and coleslaw if desired. Close the sandwich with the top half of the bun.
Serve the Nashville Hot Chicken Sandwich immediately while it's still hot and enjoy the fiery and flavorful experience!