Cut chicken breasts into bite-sized pieces. In a medium-sized mixing bowl, whisk together egg whites, cornstarch, salt and white pepper. Submerge chicken breast pieces in mixture and set aside.

Meanwhile, pour enough vegetable oil into a large pot or saucepan to cover the bottom with 2 inches of vegetable oil Heat to 350 degrees F. Working in small batches, fry chicken pieces until cooked through and golden brown, about 3 to 4 minutes. Remove from oil and drain on a paper towel-lined plate.

In a large skillet, whisk together 1/4 cup of the water with the cornstarch until dissolved. Whisk in the remaining water along with rice vinegar, brown sugar, soy sauce, hoisin sauce, garlic, ginger, and red pepper flakes. Place the skillet over high heat and bring to a simmer. The sauce will thicken upon simmering. Let simmer up to 2 minutes, then turn off the heat. Add in chicken and toss to coat.

Serve immediately. Garnish with green onions (if using).