Trim any fat from the chicken breasts and cut them into 1-inch pieces. In a medium bowl, mix together 4 tablespoons of the cornstarch with the salt and pepper. Add the chicken and toss to coat evenly.
In a large skillet or pot, heat half of the oil over medium-high heat. Once hot, add half of the chicken pieces in a single layer and cook without stirring for 3 minutes. Flip each piece and cook for an additional 3 minutes. Do not crowd the pan — if the pieces are touching, they steam instead of sear.
Remove the cooked chicken from the skillet and set aside. Repeat with the remaining oil and chicken. Pour off all but about 1 tablespoon of the oil before you build the sauce, or the finished glaze will be greasy.
Reduce the heat to medium. Add the minced garlic to the skillet and sauté for 20-30 seconds until fragrant.
Add the water, soy sauce, apple juice, chicken broth, ketchup, bourbon, vinegar, brown sugar, ground ginger, onion powder, and red pepper flakes. Stir well and bring to a boil over medium-high heat.
Leave the chicken out of the pan for this part. Reduce the heat to a steady simmer and cook the sauce uncovered for 10-15 minutes, stirring occasionally and scraping the bottom, until it has reduced by about a third and coats the back of a spoon.
In a small bowl, mix the remaining 1 tablespoon of cornstarch with 1-2 tablespoons of water to create a slurry. Stir it into the sauce and simmer for about 1 minute, until glossy.
Return the chicken and any juices from the plate to the skillet. Toss for 2-3 minutes, just until every piece is coated and heated through.
Serve hot over steamed rice.