In a mixing bowl, combine the flour, cornstarch, baking powder, salt, and black pepper.
In another bowl, whisk together the egg and water until well combined.
Dip the chicken pieces into the egg mixture, then coat them evenly in the flour mixture.
Heat the oil in a deep fryer or large skillet to 350°F (175°C).
Carefully add the coated chicken pieces to the hot oil, frying in batches until golden brown and cooked through, about 5-6 minutes per batch.
Remove the chicken from the oil and drain on paper towels to remove excess oil.
In a separate saucepan, combine the ketchup, vinegar, brown sugar, and soy sauce. Bring to a simmer over medium heat.
Stir in the cornstarch mixture and continue to simmer until the sauce thickens about 2-3 minutes.
Toss the fried chicken pieces in the sweet and sour sauce until evenly coated.
Serve the Sweet And Sour Chicken Balls hot, garnished with sesame seeds and chopped green onions if desired.