In a medium bowl or measuring cup, whisk together the soy sauce, water, rice vinegar or mirin, honey, brown sugar, garlic and ginger.
Place the chicken breasts in the instant pot in a single layer and pour the sauce over the chicken.
Close the lid, seal the pot and set the vent to “sealed”. Cook on High Pressure for 10 minutes. Let the pressure release naturally for 5 minutes, then open the vent to release the remaining pressure.
Open the lid carefully and transfer the chicken to a plate. Shred the chicken with two forks.
In a small bowl, mix the cornstarch and water to make a slurry.
Turn the Instant Pot to sauté.
Once the sauce in the Instant Pot begins to simmer, slowly stir in the cornstarch slurry. Continue whisking for 1-2 minutes to make sure the sauce is smooth and without any lumps. The sauce will begin to thicken.
Transfer the shredded chicken to the instant pot and stir to coat the chicken with the sauce. Turn the Instant Pot off
Serve garnished with green onions and toasted sesame seeds over rice (if desired).