Add soy sauce, brown sugar, chicken stock, and pepper to the insert of an instant pot or electric pressure cooker. Stir.
Add the chicken breasts into the liquid.
Place lid on the pressure cooker and lock. Make sure the pressure valve is set to keep steam inside.
For fresh chicken breasts: Use poultry setting and cook on high pressure for about 10 minutes. When instant pot has finished, use a fork or utensil to do a quick release.
For frozen chicken breasts: Use poultry setting and cook on high pressure for about 15 minutes. When instant pot has finished, use a fork or utensil to do a quick release. Make sure your frozen breasts are separated, not in one big ball of ice or it will take longer to come to pressure and longer to cook.
Remove chicken from pot and slice. Drizzle with sauce and serve as desired.
Video
Notes
Make sauce for the chicken: remove the chicken from the sauce. Mix 1 tablespoon cornstarch with 1 teaspoon water. Turn the instant pot to sauté and add the cornstarch mixture. Cook, stirring, until it boils and thickens.