To make the meatballs: Preheat oven to 400F. In a large bowl, combine ground chicken, panko, egg, ginger, garlic, scallions, soy sauce, and sesame oil. Shape into 1½-inch meatballs and transfer to a greased baking sheet. Bake for 12-15 minutes, or until the meatballs are browned and cooked through.
To make the sauce: In a small saucepan, combine hoisin sauce, rice vinegar, soy sauce, sesame oil, garlic and ginger, and bring to a boil. Remove from heat and set aside.
When the meatballs are done, discard any cooking liquid and transfer the meatballs to a mixing bowl. Add sauce and stir until the meatballs are well-coated with the sauce.
Serve warm, garnished with sesame seeds and scallions if using.