Stir bread crumbs, salt, 1/4 teaspoon black pepper, onion powder, and paprika together in a shallow bowl. Pour melted butter into a separate bowl.
Dip each chicken piece into melted butter, press into bread crumb mixture to coat, shaking off any excess. Place coated chicken pieces and onion in a 9x13-inch baking dish.
Bake in the preheated oven until chicken is no longer pink in the center and the juices run clear, 20 to 30 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Transfer chick to a plate and pour any pan drippings into a saucepan.
Place saucepan with pan drippings over medium-high heat and stir in flour. Add sour cream and black pepper; cook and stir until heated through, 1 to 2 minutes. Stir in chives and enough milk to reach desired consistency. Top chicken and onions with sour cream mixture.