In a zip-top bag, combine 3/4 cup of buttermilk, pickle brine, hot sauce, kosher salt, black pepper, and garlic powder. Add chicken thighs, seal the bag, and massage the marinade into the chicken. Refrigerate for at least 2 hours, or ideally overnight.

While the chicken marinates, set up your dredging station. In one bowl, mix together all-purpose flour, cornstarch, cayenne pepper, and paprika. In another bowl, whisk the remaining buttermilk and eggs until smooth.

Remove the marinated chicken from the refrigerator and drizzle off excess marinade. Dip each piece first into the egg mixture, then coat in the flour mixture, pressing lightly to ensure an even coating.

In a Dutch oven, heat vegetable oil to 350°F over medium-high heat. Fry the chicken in batches for 3-4 minutes per side, until golden brown and the internal temperature reaches 165°F.

Toast brioche buns until golden brown. Spread mayonnaise on each bun and layer fried chicken pieces and dill pickles, topping with the other half of the bun.