Marinate the thighs with the buttermilk, hot sauce and 1 teaspoon salt in the refrigerator for 4 to 12 hours.
Whisk the flour, paprika, garlic powder and remaining salt in a shallow pan.
Coat each thigh in the flour, dip it back into the marinade and coat it in flour a second time for a rough crust.
Rest the coated thighs on a rack for 15 minutes while heating the oil to 325°F in a Dutch oven.
Fry in two batches for 6 to 8 minutes per side, keeping the oil between 315°F and 330°F, until the chicken reaches 165°F.
Carefully whisk 1/2 cup hot frying oil with the cayenne, brown sugar, chili powder and black pepper in a heatproof bowl.
Brush the hot spice oil over both sides of each drained thigh and rest for 5 minutes before serving.