Transfer coated chicken to plate and repeat with remaining chicken. Toss half of chicken with egg whites until well coated, then dredge chicken in cornstarch mixture, pressing to adhere. Remove chicken from marinade and pat dry with paper towels. Combine cornstarch, flour, baking soda, and remaining hoisin mixture in second shallow dish until mixture resembles coarse meal. Add 2 cups hoisin mixture and simmer, whisking constantly, until dark brown and thickened, about 2 minutes.
Cook garlic, ginger, and pepper flakes until fragrant, about 1 minute. Heat oil in wok or large skillet over medium heat until shimmering.Combine 6 tablespoons hoisin mixture and chicken in zipper-lock bag refrigerate for 30 minutes.
For the marinade and sauce: Whisk hoisin, vinegar, soy sauce, sugar, cornstarch, and water in bowl.If the sauce is too thick in step 5, whisk in 1 tablespoon of water before adding the crispy chicken. In step 4, the fried chicken pieces can be held in a 200-degree oven for up to 30 minutes before being added to the sauce (if held any longer, they will lose their crispness). 6 boneless, skinless chicken thighs (about 1 1/2 pounds) cut into 1-inch pieces.
