Make the Marinade: Whisk together the hoisin sauce, white vinegar, soy sauce, sugar, cornstarch, and water in a bowl. Place 6 tablespoons into a zip-lock storage bag with the chicken; seal and refrigerate for at least 30 minutes. Set aside the remaining marinade.
Make the Sauce: Heat 1 tablespoon of vegetable oil in a skillet over medium heat until shimmering. Sauté the garlic, ginger, and red pepper flakes until fragrant. Add 2 cups of the marinade to the skillet and simmer until thickened. Remove from heat.
Prepare the Coatings: Whisk the egg whites until foamy; set aside. Combine remaining cornstarch, flour, and baking soda in a shallow dish.
Prepare the Chicken: Remove chicken from the refrigerator and pat dry. Toss half in egg whites, then coat in cornstarch mixture. Repeat with remaining chicken.
Cook the Chicken: Heat vegetable oil in a Dutch oven over medium-high heat until 350 degrees. Fry half the chicken until golden brown, about 3 minutes. Transfer to a paper towel-lined plate. Repeat with remaining chicken.
Finish: Warm the sauce over medium heat, then add fried chicken. Toss to coat and serve garnished with green onions.
