Place boneless chicken thighs in a container and pour 2 cups buttermilk over them, ensuring each piece is fully submerged. Marinate for at least 2 hours or overnight. The buttermilk tenderizes the meat and helps seasoning penetrate deeper.

Combine 2 cups all-purpose flour, 1 tsp paprika, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp black pepper, and salt to taste in a shallow bowl. Whisk together thoroughly to ensure even distribution of spices.

Cut 4 large peeled potatoes into evenly-sized chunks. Boil in salted water for 12-15 minutes until fork-tender. Drain well and return to pot. Add 4 tbsp butter and 1/3 cup milk, then mash until creamy with desired consistency. Season with salt and pepper.

Trim ends off 1 cup green beans and rinse. Boil in salted water for 4-5 minutes until tender-crisp. Drain well and set aside.

Pour vegetable oil into a large skillet or Dutch oven to reach 2-3 inches deep. Heat to 350 degrees Fahrenheit using a thermometer. This temperature ensures a golden crispy exterior without burning while keeping the inside juicy.

Remove marinated chicken thighs from buttermilk and let excess drip off. Dredge thoroughly in seasoned flour, pressing gently to adhere. Carefully place into hot oil, frying 2 at a time. Fry for 12-14 minutes, turning halfway through, until coating is deep golden brown and internal temperature reaches 165 degrees Fahrenheit. Transfer to paper towel-lined plate to drain.

Arrange fried chicken on a plate alongside mashed potatoes and green beans. Sprinkle fresh parsley over everything. Serve immediately while chicken is hot and crispy.