Dice your chicken into small, bite-sized cubes. Season them lightly with salt and pepper. Crack the egg into a bowl and whisk it. Dip each chicken piece into the egg, then coat generously with a mix of cornstarch and flour.
Heat oil in a pan over medium-high heat. A depth of about two inches is best for frying. Fry the coated chicken pieces in batches. Cook for 4-5 minutes until crispy and golden. Remove them from the pan and set aside on a paper towel-lined plate to drain excess oil.
While the chicken cools, grab a saucepan. Combine the orange juice, soy sauce, rice vinegar, brown sugar, ginger, and garlic. Heat the mixture on medium, stirring often. Add the chili paste (if using) for a little kick. To thicken, mix the cornstarch with water and pour it into the saucepan. Cook until the sauce becomes thick and glossy.
Add the fried chicken pieces to the orange sauce and toss until every piece is coated perfectly. Allow it to simmer for 3 minutes to soak up all that flavor.