Thoroughly rinse the chicken, then cover all the pieces with 4 cups of the buttermilk in a large bowl. Cover and soak in the fridge overnight, or up to 24 hours. My mom always did it this way. When you're ready to fry the chicken, remove the bowl from the fridge and let it sit on the counter for 30 minutes, just to take off the chill.
In the meantime, preheat the oven to 350 degrees F.
To mix up the breading, place the flour, seasoned salt, cayenne if using (extra if you like heat), paprika, black pepper and thyme in a very large bowl and stir together well.
In a small bowl, combine the remaining 1/4 cup buttermilk and the milk. Pour the milk mixture into the flour mixture and, with a pastry cutter or fork, gradually mix until there are little lumps throughout. This will adhere to the chicken and make for a crispier breading. If necessary, add a little more flour or milk to the bowl in order to make it slightly lumpy.
Heat 1 1/2 inches of oil in a deep skillet or Dutch oven over medium-high heat until a deep-fry thermometer inserted in the oil reaches 365 degrees F. Lower the heat slightly, if necessary, to keep the oil from getting hotter.
Working in batches, thoroughly coat each buttermilk-soaked chicken piece with the breading, pressing extra breading onto the chicken if necessary. Place the breaded pieces on a plate.
Add the chicken to the oil three or four pieces at a time. Make sure they aren't sticking together, then cover the pan and fry for 5 to 7 minutes, checking occasionally to make sure the chicken isn't getting too brown. Turn the pieces over, cover again and cook 3 to 5 minutes more. All the while, monitor the temperature of the oil to make sure the chicken doesn't burn. Keep in mind that we'll finish cooking the chicken in the oven, and it will continue to brown.
Place the fried pieces on a baking sheet and continue frying the rest of the chicken. When all the chicken has been fried, bake the chicken for 15 minutes to finish the cooking process. Sometimes I'll cut into the thicker part of one of the larger pieces, just to make sure the chicken is cooked through. If any pink (juice or meat) is visible, the chicken needs to continue cooking in the oven.
I like to serve fried chicken with buttermilk biscuits and a spread of butter and honey: Begin with 1/4 cup honey. Add an equal amount of softened butter. Mix together with a fork. Serve in a small bowl alongside the biscuits.