Season the diced chicken evenly with salt and pepper on a large shallow plate; let sit for a couple minutes.
Set the Instant Pot to Sauté and heat the canola oil. When the oil is hot, add the chicken in a single layer and brown quickly on all sides, working in batches so you do not overcrowd the pot. Transfer browned chicken to a plate after each batch and repeat until all chicken is browned.
While the chicken browns (or after), whisk together the sauce ingredients in a bowl: buffalo hot sauce, ketchup, minced garlic, grated ginger, chili paste (if using), brown sugar, rice vinegar, red pepper flakes, soy sauce, and 1/3 cup water.
Pour the sauce into the Instant Pot and use a spatula to scrape any browned bits from the bottom. Add all the browned chicken back into the pot and stir to coat.
Press Cancel to stop Sauté. Lock the lid, set the steam release to Sealing, press Pressure Cook (Manual) and set the time to 5 minutes. When the cook time ends, perform a quick release.
Use a slotted spoon to remove the cooked chicken to a large plate and set aside.
Set the Instant Pot to Sauté and bring the sauce in the pot to a simmer. In a small bowl, whisk together the cornstarch and cold water until smooth, then whisk the slurry into the simmering sauce. Cook, stirring, until the sauce thickens.
Return the cooked chicken to the pot and stir until all pieces are evenly coated in the thickened sauce.
Garnish with chopped green onions and sesame seeds, then serve immediately.