In a medium-size mixing bowl, whisk together the hoisin sauce, rice vinegar, soy sauce, brown sugar, minced garlic, ground ginger, red pepper flakes, and 2 teaspoons of cornstarch until well blended. Set this sauce aside.

In a large ziplock bag, combine the flour, remaining 3/4 cup of cornstarch, salt, and black pepper. Shake the bag thoroughly to mix the ingredients.

In a medium-size mixing bowl, add the cubed chicken and the beaten egg whites. Toss the chicken pieces to ensure an even coating.

In a large skillet, heat 1/2 cup of vegetable oil over medium-high heat until shimmering.

Using a large slotted spoon, carefully scoop the chicken pieces, allowing the excess egg whites to drip off. Drop the chicken into the ziplock bag with the flour mixture, then shake until the pieces are evenly coated.

Once the oil is hot, fry the chicken in batches to avoid overcrowding the pan. Cook each batch until golden brown and chicken is fully cooked through.

Remove the fried chicken pieces and place them on a sheet pan lined with paper towels to drain the excess oil.

After all the chicken has been fried, wipe out the skillet, removing excess oil. Pour the prepared sauce into the skillet and simmer until it starts to thicken.

Add the fried chicken back into the skillet and toss to coat all the pieces evenly with the sauce.

Serve the General Tso's Chicken immediately with white or brown rice, and optionally garnish with sliced green onions.