Add chicken thighs and 3 tablespoons of cornstarch to a medium mixing bowl. Toss until the chicken is evenly coated.
Heat the oil in a large wok or nonstick skillet over medium-high heat. Working in two batches, add the chicken to the pan in a single layer, as much as possible. Cook without disturbing for 1 minute so it can develop a crust, flip, and cook for another minute. Transfer the thighs to a cutting board and repeat. Chop the chicken into bite-size pieces.
Return all of the chicken to the pan. Stir until browned and cooked through, about 8 minutes. Transfer the chicken to a plate and set aside.
Whisk together the remaining tablespoon of cornstarch, soy sauce, chicken broth, hoisin sauce, bourbon, and brown sugar in a medium mixing bowl. Add the garlic and ginger to the wok and sauté for 20 to 30 seconds until fragrant. Pour the sauce into the pan and bring it up to a simmer. Allow the sauce to simmer for several minutes until it starts to thicken.
Add the cooked chicken back into the pan. Turn the heat down to medium and cook for 2 to 3 minutes. Check the seasoning and add salt and pepper to taste. Once the chicken is heated through and the sauce has reached your desired thickness, it's ready.
Serve over cooked white rice and garnish with scallions before enjoying warm.