Place chicken thighs and drumsticks in a bowl and pour enough buttermilk over chicken to cover. Cover and refrigerate for 12 to 24 hours.

Combine flour, cornmeal, salt, granulated onion, granulated garlic, thyme, paprika, monosodium glutamate, baking powder, and cayenne pepper in a large, wide bowl.

Remove chicken from buttermilk and shake off excess; discard buttermilk. Pat chicken dry with paper towels. Dip chicken in beaten egg whites, then press in flour mixture to coat. Allow coated chicken to rest on a wire rack for 20 to 30 minutes.

Meanwhile, fill a cast iron skillet or deep fryer to about 1/3 full of oil and heat to 350 degrees F (175 degrees C). Preheat the oven to 250 degrees F (120 degrees C).

Fry chicken in hot oil in batches until golden brown, no longer pink in the center, and the juices run clear, 8 to 10 minutes per side. An instant-read thermometer inserted into the center of chicken near the bone should read at least 165 degrees F (74 degrees C). Transfer fried chicken to a wire rack or a paper towel-lined tray to drain. Keep fried chicken warm in the preheated oven while frying remaining pieces.