Whisk to combine egg, cornstarch, and soy sauce in a mixing bowl. Once you have a smooth batter add the chicken and let sit for half an hour.
In a second mixing bowl cornstarch, flour baking soda, and salt. Whisk combine.
Heat oil in, preferably, a heavy bottom frying pan or a wok. You know the oil is hot enough when you drop in a bit of batter and it sizzles. You want a temperature between 325°F and 375°F.
Roll each piece of battered chicken in the dry mixture. Shake off excess and add it to the hot oil. Repeat until the pan is full but not over crowded. Fry until crispy and place on a paper towel lined plate. Repeat in batches with the remaining pieces of chicken.
Add in a frying pan add oil. When it begins to shimmer add garlic and ginger. Lightly fry for about 10 seconds constantly stirring.
Add soy sauce, sherry if desired, brown sugar, white sugar, cornstarch, vinegar, sesame oil, and bullion powder. Stir until everything is well combined including the sugar. Simmer until it develops a thick syrup-like texture. Let cool slightly.
Place the chicken in the sauce, sprinkle sesame seeds, and toss to coat.