Combine buttermilk, salt, and pepper in a large bowl. Add chicken pieces and marinate at room temperature for 30 minutes.
Preheat the oven to 450 degrees F (230 degrees C). Line a rimmed baking pan with foil and place in the oven.
Combine cornstarch, panko, smoked paprika, Greek seasoning, onion powder, poultry seasoning, salt, and pepper in a large resealable bag.
Remove chicken legs from buttermilk, drain, and dip legs one at a time into the dry coating mixture.
Remove pan from the oven with pot holders and add 1 tablespoon melted butter to the pan. With a pastry brush, spread melted butter all over the pan.
Lay coated chicken legs into the pan. Pour half of the melted butter over chicken legs.
Roast in the preheated until chicken is no longer pink at the bone and the juices run clear, about 40 minutes, turning over at the halfway point. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C). Drizzle with remaining melted butter and serve immediately.