In a mixing bowl, whisk together egg white, 2 tablespoons of the cornstarch, 2 tablespoons of the dry sherry, 1 tablespoon soy sauce, and chili powder; whisk until well combined.
Add the chicken and toss to coat.
Cover and refrigerate 2 hours.
In a mixing bowl, whisk 1 tablespoon cornstarch with 1 tablespoon of the chicken stock; whisk until smooth.
Add the remaining chicken stock, 1 tablespoon dry sherry, 1 tablespoon soy sauce, rice vinegar, and sugar; whisk until combined; set aside.
In a large pot, heat vegetable oil to 350 degrees.
Remove chicken from the marinade and slide into the hot oil.
Fry the chicken, turning it once, until golden brown and cooked through; about 2 minutes.
Remove and drain on paper towels.
In a large saucepan or wok, heat 1 tablespoon vegetable oil.
Add the chile peppers, sliced red bell pepper and broccoli; cook until tender.
Add garlic, ginger, red pepper flakes and 1/2 cup green onions.
Continue to cook for a few seconds; add the chicken stock sauce, bring to a boil, and cook, stirring, until the sauce thickens, about 1 minute.
Remove from heat.
Arrange cooked chicken on a platter and pour the sauce over it.
Serve over rice and garnish with green onions and/or sesame seeds.