Preheat the oven to 500ºF and spray a mini-muffin pan or baking dish with cooking spray.
In a large bowl, stir together the milk and Panko breadcrumbs and let them sit for 5 minutes.
Add the ground chicken, scallions, garlic, ginger, soy sauce and egg to the bowl with the breadcrumbs. Use your hands to mix the ingredients just until combined. (See Kelly’s Notes.)
Shape the meat mixture into balls using about 3 tablespoons of meat per meatball. Place the shaped meatballs into the prepared pan and bake for 15 minutes, or until the meatballs are fully cooked.
While the meatballs are baking, whisk together the hoisin sauce and blackberry jam in a small sauce pan over medium heat.
Remove the meatballs from the oven and either transfer them in batches into the glaze, tossing to coat, or brush the meatballs with the glaze. Place one or two meatballs in each leaf of lettuce, sprinkle with sesame seeds and serve.