In a large mixing bowl, combine 2 cups buttermilk with 2 teaspoons salt, 1 teaspoon black pepper, 1 teaspoon garlic powder, 1 teaspoon paprika, and 1/2 teaspoon cayenne pepper. Whisk until the salt fully dissolves.

Pat 8 chicken thighs dry with paper towels, then place them in a shallow dish or baking pan. Pour the buttermilk mixture over the thighs, making sure each one is fully submerged or well coated. Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

In a shallow bowl, combine 2 cups all-purpose flour, 2 teaspoons salt, 1 teaspoon black pepper, 1 teaspoon garlic powder, 1 teaspoon paprika, 1/2 teaspoon onion powder, and 1/2 teaspoon cayenne pepper. Mix everything together with a fork or whisk.

Pour 2 to 3 quarts of vegetable oil into a heavy-bottomed pot or Dutch oven, filling it to about 2 inches up the sides. Clip a candy or deep-fry thermometer to the side of the pot and heat the oil to 350 degrees Fahrenheit.

Remove the chicken thighs from the marinade one at a time and place them on a paper towel-lined plate. Pat each thigh dry on both sides, removing excess buttermilk.

Working with one thigh at a time, place it in the shallow bowl with the flour mixture and coat all sides thoroughly, pressing the flour gently onto the meat. Shake off excess flour and place the coated thigh on a clean plate or wire rack. Let the coated chicken sit for about 5 minutes before frying.

Carefully place 3 to 4 coated thighs into the hot oil at 350 degrees Fahrenheit. Fry for 12 to 15 minutes, turning halfway through the cooking time, until the coating is deep golden brown and the internal temperature reaches 165 degrees Fahrenheit.

Using a slotted spoon or tongs, remove the fried chicken thighs from the oil and place them on a wire rack set over a baking sheet or on a plate lined with paper towels. Let the chicken rest for 3 to 5 minutes before serving.

Repeat the frying process with the remaining coated chicken thighs, frying 3 to 4 pieces at a time. Check your oil temperature before each batch to ensure it returns to 350 degrees Fahrenheit before adding more chicken.