Combine the cubed 4 boneless skinless chicken breasts, and ¼ cup cornstarch in a bowl or plastic bag. Toss until the chicken is completely coated in the cornstarch.
In a medium-sized skillet over medium-high heat add 1 tablespoon vegetable oil. Cook the chicken until it is lightly brown but not cooked through. Place the chicken in the bottom of a slow cooker.
In a small mixing bowl, whisk together ½ cup hoisin sauce, 2 tablespoons soy sauce, ½ cup brown sugar, 3 cloves garlic, 3 tablespoons rice wine vinegar, 1 teaspoon sesame oil, ¼ teaspoon dry ginger, and ½ teaspoon crushed red pepper. Pour on top of the chicken in the slow cooker.
Cook on low heat for 3-4 hours or until chicken is cooked throughout.
Serve over rice and garnish with green onions, sesame seeds, and red pepper flakes if desired.
