Preheat oven to 400°F and line a baking sheet with parchment.
Combine ground chicken, panko, egg, garlic, ginger, and 2 tbsp soy sauce and roll into 1.5-inch balls.
Bake for 18-20 minutes until cooked through.
While meatballs bake, combine 1/3 cup soy sauce, mirin, brown sugar, and rice vinegar in a saucepan over medium heat, stir in cornstarch slurry, and cook 3-4 minutes until thickened.
Toss baked meatballs in teriyaki glaze and place back in oven for 3 minutes.
Garnish with sesame seeds and green onions and serve.