Whisk together the buttermilk, 1 teaspoon of salt, and 1/2 teaspoon of black pepper in a shallow dish. Add the chicken thighs, turning to coat. Cover and refrigerate for at least 30 minutes to help tenderize and flavor the meat.
Meanwhile, in a separate bowl, combine the flour, paprika, garlic powder, onion powder, cayenne, and a pinch of salt. Mix well to create a seasoned coating for the chicken.
Fill a deep skillet or Dutch oven with about 1 inch of oil and heat to 350°F (175°C). If you don’t have a thermometer, drop a small bit of flour into the oil; if it sizzles steadily, you’re ready.
Remove the chicken from the buttermilk, letting excess drip off. Dredge each piece in the seasoned flour, pressing gently to ensure a thick, even coat.
Fry the chicken in batches, turning once, until golden brown and crisp and an internal thermometer reads 165°F (74°C), about 6–8 minutes per piece depending on thickness. Transfer cooked chicken to a rack lined with paper towels to drain briefly.
While the chicken rests for a couple of minutes, stir together the mayonnaise and pickle juice (if using) for a tangy spread. Toast the brioche buns briefly on the hot side of the pan or in a toaster.
Spread a generous layer of the mayo on the bottom bun, layer with dill pickle slices, place the hot fried chicken on top, and crown with the top bun. Serve immediately for the best crunch.