In a large mixing bowl, mix the milk with the panko breadcrumbs and let sit for 5 minutes. Add the ground pork, beaten egg, minced garlic, minced ginger, chopped green onions, 2 tablespoons soy sauce, the salt, and black pepper. Mix until just combined.
Gently form the mixture into small meatballs about 1.5 inches in diameter and place them on a baking sheet lined with parchment paper.
Preheat your oven to 400°F. Bake the meatballs for 10 to 15 minutes until they are golden on the outside and cooked through.
While the meatballs bake, combine the remaining soy sauce, hoisin sauce, rice vinegar, sesame oil, and red pepper flakes in a small saucepan. Heat the sauce over medium-low heat for 3 to 4 minutes until it slightly thickens.
Once the meatballs are done, transfer them to the saucepan and gently toss to coat. Serve warm, garnished with extra green onions and sesame seeds.