Begin by combining the chicken pieces with buttermilk, salt, and black pepper in a large mixing bowl. Ensure the chicken is well-coated, then cover the bowl with plastic wrap and refrigerate for at least 30 minutes, or up to 4 hours.

While the chicken marinates, gather a shallow dish and whisk together the all-purpose flour, cornstarch, paprika, garlic powder, onion powder, and optional cayenne pepper.

After marinating, remove each piece of chicken from the buttermilk, allowing the excess to drip off. Dredge the chicken in the flour mixture until fully coated.

In a heavy pot or deep fryer, pour in enough vegetable oil to submerge the chicken pieces (about 4-6 cups). Heat the oil to 350°F (175°C).

Carefully add the chicken pieces to the hot oil in batches, taking care not to overcrowd the pot. Fry the chicken for about 6-8 minutes until golden brown and cooked through.

In a saucepan over medium heat, melt 2 tablespoons of unsalted butter. Once melted, whisk in flour to create a roux, cooking for about 1-2 minutes until lightly golden.

Gradually add the milk while continuously whisking to avoid lumps. Cook until the mixture thickens, then stir in the shredded sharp cheddar cheese until melted and creamy.

To present your Ultimate Crispy Fried Chicken with Creamy Sauce, arrange the freshly fried chicken on a serving platter. Generously drizzle the creamy sauce over the chicken and garnish with fresh chives or green onions.