1. For the marinade and sauce: Whisk hoisin, vinegar, soy sauce, sugar, cornstarchand water in bowl.
2. Combine 6 tablespoons hoisin mixture and chicken in zipper-lock bag; refrigerate 30 minutes.
3. Heat oil in large skillet over medium heat until shimmering.
4. Cook garlic, ginger, and pepper flakes until fragrant, about 1 minute.
5. Add 2 cups hoisin mixture and simmer, whisking constantly, until dark brown and thickened, about 2 minute.
6. Cover and keep sauce warm.
7. For coating and frying: Whisk egg whites in shallow dish until foamy.
8. Combine cornstarch, flour, baking soda, and remaining hoisin mixture in second shallow dish until mixture resembles course meal.
9. remove chicken from marinade and pat dry with aper towels.
10. Toss half of the chicken with egg whites until well coated, then dredge chicken in cornstarch mixture, pressing to adhere.
11. Transfer coated chicken to plate and repeat with remaining chicken.
12. Heat oil in Dutch oven over medium-high heat until oil registers 350 degrees.
13. Fry half of the chicken until golden brown, about 3 minutes, turning each piece halfway through cooking.
14. Transfer chicken to paper towel-lined plate.
15. Return oil to 350 degrees and repeat with remaining chicken.
16. To serve: Warm sauce over medium-low heat until simmering, about 1 minute.
17. Add crispy chicken and toss to coat.
18. Serving over rice or noodles would be awesome!