Do ahead: In a large bowl, mix together mochiko flour, all purpose flour, cornstarch, sugar, garlic, eggs, scallions, sesame oil, oyster sauce, soy sauce, salt, and pepper, whisking to combine well.
Towel-dry the chicken and use a fork to poke throughout all chicken pieces.
Add chicken to the bowl of sauce mixture, coating completely. Cover and marinate 12 hours or overnight.
If desired, make the Optional Dipping Sauce by combining sauce ingredients and whisking together. Cover and set aside until ready to use.
Line a large baking sheet with paper towels. Heat a generous amount of oil in a large heavy pan - oil should go deep enough to completely submerge chicken pieces.
Once oil reaches 350F, fry chicken pieces in batches until golden brown on all sides - do not over crowd. Adjust heat as needed to keep oil temp as close to 350F as possible. Use tongs to turn pieces over as needed. Once chicken is golden and floats to top, it's done.
Drain cooked chicken on plate lined with paper towels. Continue frying in batches. Serve warm, garnished with chopped scallions and sesame seeds if desired. Serve with or without dipping sauce.
Notes
Mochiko is a brand of "glutinous" or "sweet" rice flour found in the Asian section of major grocery stores. Though its name suggests otherwise, it does not contain gluten, nor does it taste sweet.
Do not substitute rice flour for mochiko flour, as the two are not interchangeable. See original article for details.
Gluten-free: To make this recipe gluten-free, replace the all-purpose flour with extra mochiko flour and use gluten-free tamari sauce in place of soy sauce.
Don't overcrowd the pan. Fry the chicken in batches to ensure even cooking and to prevent the oil temperature from dropping too much.
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