For the Chicken Rice Balls, cook sushi rice as directed on package, using 2 cups water. Fluff with fork. Set aside to cool slightly while preparing filling.
To make the marinade, mix plum sauce, mirin, plum vinegar, soy sauce, ginger and Japanese Seven Spice in large bowl. Remove 3 tablespoons of the marinade to small bowl; set aside. Add chicken to remaining marinade; cover. Refrigerate 30 minutes.
Ume plum vinegar or umeboshi vinegar is a Japanese condiment with salty, sour and sweet flavors. It is a by-product of pickling Japanese plums. It can be found in natural or organic food stores, Asian markets or online.
To cook the chicken, heat oil in medium skillet on medium-high heat. Remove chicken from marinade. Add chicken to skillet; cook and stir 2 minutes. Reduce heat to medium. Add reserved marinade; cook 3 to 5 minutes longer or until chicken is browned and cooked through, turning frequently.
To form the rice balls, wrap 1 chicken piece in 1 bok choy leaf. Scoop about 1/3 cup warm rice. Spoon 1/2 of the rice into palm of hand and make a deep indentation in rice. Place bok choy-wrapped chicken in indentation. Top with remaining rice. Using hands, shape rice around chicken, forming into a triangle or a ball, to enclose filling. Place on wax paper or parchment paper-lined tray. Repeat with remaining chicken, bok choy and rice. Cover with plastic wrap. Freeze 30 minutes or until rice balls are slightly frozen.
Keep a bowl of cold water nearby when preparing rice balls. If your hands get too sticky while forming the rice balls, wet them in the cold water and continue forming the rice balls.
Meanwhile for the Creamy Plum Vinegar Ginger Sauce, mix all ingredients in medium bowl until well blended. Cover. Refrigerate until ready to serve.
Remove rice balls from freezer. Dip each rice ball in beaten egg then roll in panko to coat evenly. Return to tray. Heat 1 inch of oil to 325°F to 350°F in large skillet on medium heat. Add 1/2 of the rice balls; cook 5 minutes or until browned, turning occasionally. Drain on paper towels. Repeat with remaining rice balls. Serve rice balls with Creamy Plum Vinegar Ginger Sauce.