Step 1: Marinate the Chicken

1. In a large bowl, combine the buttermilk, garlic powder, smoked paprika, and cayenne pepper.
2. Add the chicken thighs or breasts and ensure they are fully submerged.
3. Cover and refrigerate for at least 1 hour, or up to overnight for maximum flavor.

Step 2: Dredge the Chicken

1. In another bowl, mix the flour, salt, pepper, and a pinch of cayenne.
2. Remove the chicken from the buttermilk marinade, letting excess drip off.
3. Coat each piece of chicken in the flour mixture, pressing lightly to adhere. Set aside on a plate.

Step 3: Fry the Chicken

1. Heat about 2 inches of oil in a skillet or Dutch oven to 350°F (175°C).
2. Fry the chicken in batches, 4-5 minutes per side, until golden brown and cooked through. The internal temperature should reach 165°F (74°C).
3. Transfer the chicken to a wire rack to drain.

Step 4: Make the Hot Sauce

1. In a small saucepan, heat vegetable oil over medium heat.
2. Stir in cayenne pepper, smoked paprika, garlic powder, brown sugar, chili powder, and a pinch of salt.
3. Remove from heat and whisk until smooth. Brush the sauce generously over the fried chicken.

Step 5: Assemble the Sandwiches

1. Toast the brioche buns lightly, if desired.
2. Spread mayonnaise on the bottom bun (optional).
3. Place a piece of spicy chicken on the bun.
4. Top with pickles and coleslaw (if using).
5. Add the top bun and serve immediately.