Place the chicken breasts in the slow cooker pot in a single layer to ensure even cooking.
In a small bowl, combine the soy sauce, honey, vinegar, brown sugar, cornstarch, sesame oil, minced garlic, ground ginger, and black pepper. Whisk until well blended and the cornstarch is completely dissolved.
Pour the homemade teriyaki sauce over the chicken, making sure all pieces are coated.
Cover and cook on LOW for 4-5 hours, until the chicken is fully cooked to an internal temperature of 165°F and tender enough to shred easily.
Remove the chicken from the slow cooker and place on a cutting board. Shred with two forks or slice into strips, depending on your preference.
Return the shredded or sliced chicken back to the slow cooker and stir it into the thickened teriyaki sauce until well coated.
Serve over steamed rice, garnished with sliced green onions and a sprinkle of sesame seeds.
Notes
Don't overcook: Chicken breast can become dry if cooked too long. Stick to the recommended time and check for doneness at the earlier end of the range.
Single layer matters: Arrange chicken in a single layer if possible to ensure even cooking throughout.
Cornstarch dissolving: Make sure the cornstarch is completely dissolved in the sauce mixture before pouring over the chicken to avoid lumps.
Check halfway: If possible, give the sauce a gentle stir halfway through cooking to ensure even coating and prevent burning along the edges.
Low and slow: For the most tender results, use the LOW setting rather than HIGH. The gentle, slow cooking allows the chicken to become perfectly tender.
Cut vs. shred: If you prefer chicken pieces rather than shredded chicken, you can cut the chicken into chunks before cooking or slice it after cooking instead of shredding.