Assemble: In the insert of a 6 quart Instant Pot, stir together soy sauce, ⅓ cup water, brown sugar, ginger, garlic, and sesame oil until sugar is dissolved. Add chicken breasts and turn to coat.
Pressure Cook: Secure the lid and set the valve to SEALING. Pressure cook on HIGH for 8 minutes.
Quick Release: When the cooking cycle ends, immediately perform a quick release. Carefully turn the valve to VENTING. Once the pin drops, remove the lid.
Check & Rest: Verify chicken has reached 165°F (74°C) with an instant read thermometer. Transfer chicken to a cutting board and let rest for 5 minutes.
Thicken Sauce: Select the SAUTÉ function. In a small bowl, whisk 2 tablespoons water with cornstarch to make a slurry. Once the sauce is simmering, slowly whisk in the slurry. Cook for 1-2 minutes, stirring constantly, until the sauce thickens and becomes glossy.
Remove Pot: Use oven mitts to carefully lift the inner pot out of the Instant Pot base to stop the cooking.
Slice & Toss: Slice the rested chicken against the grain into strips or cubes. Return the chicken to the pot and toss to coat evenly in the sauce.
Serve: Serve immediately over rice or noodles, garnished with sesame seeds and green onions if desired.