In a medium saucepan, bring the soy sauce, mirin, and sugar to a boil. Reduce the heat to maintain a simmer and cook until the liquid has reduced by one-third, about 20 minutes. Stir in the mizuame and cook until the mizuame dissolves, about 5 minutes. Remove the pan from the heat and let the glaze cool.
In a large bowl, mix the ground chicken with the salt, pepper, scallions, and ginger until thoroughly combined. Form the meat into 20 oblong meatballs, about 1 1?2-oz. each, and place the meatballs on a lightly greased foil-lined baking sheet.
Heat the broiler and broil the meatballs, turning and basting every 2 minutes with the glaze, until the meatballs are cooked through and dark golden brown, about 8 minutes. Transfer to a platter and serve while hot.