Make the Marinade: Whisk the hoisin sauce, vinegar, soy sauce, sugar, cornstarch, and water in a bowl. Of this mixture, place 6 tablespoons into a zip lock storage bag and add the chicken; seal and refrigerate for at least 30 minutes. Set aside the remaining marinade in the bowl.
Make the Sauce: While the chicken is chilling in the marinade, heat the 1 tablespoon of vegetable oil in a large skillet over medium heat until shimmering. Sauté the garlic, ginger and red pepper flakes until fragrant. Add 2 cups of the hoisin marinade to the skillet and simmer, whisking constantly, until the mixture is dark brown and thickened. Remove from heat and cover to keep the sauce warm.
Prepare the Coatings: Whisk the egg whites in a shallow dish until foamy; set aside. Combine the cornstarch, flour, baking soda, and remaining hoisin marinade in a second shallow dish; mix until it resembles coarse meal.
Prepare the Chicken: Remove the chicken from the refrigerator and from the marinade. Pat the marinated chicken dry with paper towels. Toss half the chicken into the foamy egg whites until well coated, then dredge the chicken in the cornstarch mixture, pressing to adhere. Transfer the coated chicken to a plate and repeat with the remaining chicken.
Cook the Chicken: Heat the oil in a Dutch oven over medium high heat until the oil registers 350 degrees. Fry half the chicken until golden brown, about 3 minutes, turning each piece halfway through cooking. Transfer the cooked chicken onto a paper towel lined plate to drain. Return the oil to 350 degrees before frying again. Repeat with the remaining chicken.
Finish: Warm the sauce over medium heat until simmering. Turn off the heat and add the fried chicken pieces. Toss to coat and serve with sliced green onions to garnish, as well. White jasmine rice and steamed broccoli are favorite sides here!