Make the marinade: Mix 4 tablespoons soy sauce, brown sugar, and white wine in a small bowl and combine well with a whisk. Pour into a zip top bag (you can also just add the ingredients to a zip top bag, or marinate in the bowl). Add the chicken pieces to the marinade. Close the bag tightly being sure all pieces of chicken get coated. Marinate for at least half an hour in the refrigerator.
While the chicken is marinating, prepare the vegetables and set aside.
Heat 2 tablespoons oil in a large skillet or wok. Season chicken with salt and pepper then cook the chicken over medium-high heat until well seared on all sides, about 2-3 minutes. If chicken begins to burn, reduce heat to medium heat. Move the cooked chicken to a plate and set aside.
Make the sauce: Mix oyster sauce, 3 tablespoons soy sauce, sesame oil, chopped garlic, minced ginger, and sugar together in a small bowl. In a separate bowl, dissolve corn starch with 2 tablespoons of water and then add it to the sauce, stirring to combine well. Set aside while you cook the vegetables.
If necessary, add more oil to the pan and set heat to medium-high. Add diced peppers, zucchini, and peanuts on medium heat until crisp-tender, about 2 minutes.
Return the cooked chicken to the pan and cook everything together for another 1-2 minutes.
Pour the sauce into the pan and stir it with the chicken and vegetable mixture. Cook over low heat for about 5 minutes or until sauce is thickened and glossy and chicken is cooked to 165 degrees F.
Serve Kung Pao chicken topped with green onions, crushed red chili flakes, and hot cooked rice.
Notes
Chop veggies uniformly: Keep your bell peppers and zucchini the same size for even cooking.
High heat is your friend: Crank up the heat to get a good sear on the chicken and to keep veggies from getting watery.
Stir frequently: Moving the ingredients around constantly will prevent burning and ensure everything cooks evenly.
Consistency is key: Mix your sauce ingredients (soy sauce, vinegar, sugar, and water) in a bowl beforehand. This helps avoid a lumpy sauce situation.
Taste as you go: Don't be shy to tweak the sweet and savory levels to your liking before adding the sauce to the pan. You can add more or less sugar and soy sauce to adjust to your liking.