Make the marinade by combining the buttermilk, sour cream, Dijon mustard, salt and freshly ground black pepper in a large bowl. Mix well with a whisk. Season the chicken pieces with salt and pepper and place in the marinade. Stir everything together so the chicken is coated evenly, cover, and set aside in the fridge to marinate for 12 hours.
Fill a heavy-based pot (such a deep cast iron skillet) with canola oil and set over medium-high heat. Using a thermometer, heat the oil until 375 degrees F. Preheat the oven to 350 degrees F.
Prepare the coating by adding the all-purpose flour, salt, paprika, mustard powder, and garlic powder together in a large bowl. Mix with a whisk. Take the chicken pieces out of the marinade and dredge lightly in the seasoned flour, 1 at a time. Set the coated pieces on a wire rack over a tray as you work.
Add the chicken, 3 to 4 pieces at a time, to the hot oil, skin-side down. Cook until golden brown, 5 to 7 minutes. Increase the heat on the oil if you need to as the chicken will bring the temperature of the oil down. Remove the chicken from the pot with a wire strainer and set over a clean rack over a tray. Season with salt while still hot. Continue to cook in batches, allowing the oil to heat up again before adding the chicken to the pot each time.
Place the chicken into the oven and cook for another 8 to 10 minutes until cooked through. Allow the chicken to rest for 5 minutes before serving. Pile high in a basket for a fun presentation.