Whisk the sauce: In a bowl or measuring cup, whisk together soy sauce, brown sugar, ketchup, apple cider vinegar, bourbon, water, garlic, ginger, onion powder, smoked paprika, and red pepper flakes if using.
Add the chicken: Place the chicken pieces in your slow cooker, Instant Pot, or a large skillet. Pour the sauce over the top and stir to coat evenly.
Choose your method: Slow cooker: Cover and cook on Low for 3 to 4 hours or on High for 2 to 2.5 hours, until the chicken is tender.
Instant Pot: Seal and pressure cook on High for 6 minutes.
Let it naturally release for 5 minutes, then quick release.
Stovetop: Warm 1 tablespoon oil in a large skillet over medium-high heat. Add chicken and sauce. Bring to a simmer, then reduce heat to medium-low and cook 15 to 20 minutes, stirring occasionally, until the chicken is cooked through.
Thicken the sauce: Stir the cornstarch slurry.
Switch the slow cooker to High (lid off) and stir in the slurry. Cook 10 to 15 minutes until glossy and thick. For Instant Pot, switch to Sauté, add the slurry, and cook 2 to 3 minutes.
For stovetop, stir in the slurry and simmer 1 to 2 minutes until thickened.
Finish and serve: Taste and adjust seasoning. Add a splash of vinegar for more tang or a pinch of sugar if you want it sweeter. Serve over hot rice, and garnish with sesame seeds and green onions.