Prepare the Brine: Combine all of the brine ingredients in a large stockpot, cover, and bring to a boil. Boil for 1 minute, stirring to dissolve the salt. Remove from the heat and cool completely, then chill thoroughly in the refrigerator before using. The brine can be refrigerated for up to 3 days. Pour the brine into a large container, add the chicken pieces and refrigerate for 8 to 12 hours (don't refrigerate for much longer, or the chicken will be too salty).
Prepare the Chicken: Remove the chicken from the brine and rinse under cold water, washing off any particles from the brine stuck to the chicken. Pat dry with paper towels and let sit at room temperature for 1½ hours, or until it reaches room temperature.
Prepare the Coating and Dredging Station: Fill a large pot with at least 2 inches of frying oil, place it over medium heat and heat to 320 degrees F. Place a wire cooling rack on top of a baking sheet and place it to the side of the pot of oil to drain the cooked chicken.
Meanwhile, whisk together the flour, garlic powder, onion powder, paprika, cayenne, salt and pepper. Divide the mixture between two shallow bowls. Pour 1 quart of the buttermilk into a third shallow bowl. Add the 6 tablespoons of buttermilk to one of bowls with the flour mixture and use a fork or your fingers to toss the mixture together - all of the moisture from the buttermilk should be absorbed and the mixture should look a little shaggy. Set up the station in this order: chicken pieces, dry flour mixture, buttermilk and shaggy flour mixture, then a parchment-lined baking sheet.
Just before frying, dip the chicken thighs into the first bowl of coating, turning to coat and patting off the excess; dip them into the buttermilk, allowing the excess to run back into the bowl; then dip them into the second bowl of shaggy coating. Transfer to the parchment-lined pan.
Fry the Chicken: Carefully lower the thighs into the hot oil. Adjust the heat as necessary to return the oil to the proper temperature. Fry for 2 minutes, then carefully move the chicken pieces around in the oil and continue to fry, monitoring the oil temperature and turning the pieces as necessary for even cooking, for 11 to 12 minutes, until the chicken is a deep golden brown, cooked through (should register 170 degrees F on an instant-read thermometer), and very crisp. Meanwhile, coat the chicken drumsticks and transfer to the parchment-lined baking sheet.
Transfer the cooked thighs to the cooling rack skin-side-up and let rest while you fry the remaining chicken. Ensure that the oil has returned to 320 degrees F, and cook the chicken drumsticks for about 8 minutes, checking for doneness as you did with the thighs. When the drumsticks are done, transfer them to the cooling rack skin-side-up.
Increase the heat slightly and bring the oil up to 340 degees F. Meanwhile, coat the chicken breasts and wings. Carefully lower the chicken breasts into the hot oil and fry for 7 minutes, or until golden brown, cooked through (should register 165 degrees F on an instant-read thermometer), and crisp. Transfer to the rack, skin-side-up. Cook the wings for 6 minutes, or until golden brown and cooked through. Transfer the wings to the rack and turn off the heat. Allow the chicken to rest for about 10 minutes before serving. If made ahead, the chicken can be placed in a 400 degree F oven for a few minutes to warm up.
Brine: If you don't have time for the brine (it needs to boil, then cool to room temperature, then be completely chilled before using), you can do the following: Place 1-quart buttermilk and the chicken in a resealable plastic bag, and refrigerate overnight, then proceed with the rest of the recipe as written. You won't get as much flavor this way, but it saves quite a bit of time.
Brine in Advance: You can make the brine up to 3 days before you plan to make the fried chicken.
Chicken: Buying pre-cut chicken pieces might be a little more expensive, but will save time and be less messy than cutting a chicken into pieces.
Reheating Instructions: If you have left-overs or make this fried chicken in advance, place the chicken in a 400°F oven for about 15 minutes to warm up.