Cut chicken into 1 1/2-inch pieces and place in a quart-sized bag. Set aside in the refrigerator.
In a small saucepan pan place the soy sauce and sugar. Bring to a boil.
Reduce the heat to medium and add the garlic, sesame seed oil, and red pepper flakes if using. Simmer for 3-4minutes. Set aside to cool, about 30 minutes.
Strain the marinade into a 2-cup liquid measuring cup.
Place 1/4 cup of marinade and a 1/2 Tbsp sugar back into the saucepan. Turn on medium-low to dissolve the sugar, and then remove from the heat.
Use the remaining marinade for the chicken in the bag. Place the chicken and marinade back into the refrigerator for 15 minutes.
Start to heat your oil for deep frying. The temperature should be between 350-375℉.
Place the flour and cornstarch in a gallon-sized bag and shake to mix. After 15 minutes of marinating, strain the chicken and discard the marinade.
Place all chicken pieces in with the flour mixture and shake to coat.
When the oil is hot, place a few coated chicken pieces into the oil, not crowding your pot. Cook for 1 1/2 – 2 minutes until golden brown. Remove to paper towels and sprinkle with salt.
When all the chicken has been cooked and salted, place it in a 1.5-quart bowl. Add the reserved marinade and shake to coat the pieces.
Top with sliced green onions before serving.