Assemble- in the stainless steel inner pot of the Instant Pot, stir together the soy sauce, honey, water, mirin, garlic and ginger. Add the chicken breasts and turn to coat in the sauce.
Cook- place the lid on the Instant Pot, set the valve to the 'sealing' position, and pressure cook on high pressure for 8 minutes. As soon as the cooking cycle has completed, perform a quick pressure release.
Thicken- use tongs to transfer the chicken to a clean cutting board, and select the 'sauté' function of the Instant Pot. Stir together the cornstarch and water, then add to the pot. Stir frequently, until the sauce bubbles and thickens- around 2 minutes. Use oven mitts to remove the inner pot from the Instant Pot so that the sauce doesn't over-reduce.
Toss & serve- cut the chicken into strips or cubes, then toss with the sauce. Serve with rice and steamed broccoli, sprinkle with sesame seeds if desired.