Clean the chicken pieces and pat dry with paper towels.
Make a paste by blending the garlic, cilantro roots (or stems), and sea salt in a mini food processor or pounding them with a mortar and pestle until fine.
Combine the paste, fish sauce, oyster sauce, and ground black pepper in a large bowl or resealable bag with the chicken; mix to coat evenly.
Seal and refrigerate the chicken to marinate for 4–6 hours or preferably overnight.
When ready to cook, heat enough oil in a large pot or deep fryer to fully submerge the chicken and bring to 350–375°F (175–190°C).
While the oil heats, mix the all-purpose flour, cornstarch, and rice flour in a bowl or resealable bag.
Add marinated chicken pieces to the flour mixture and toss or shake to coat each piece evenly, then shake off any excess flour.
Carefully lower the coated chicken into the hot oil in batches; fry until golden brown and crispy and the internal temperature reaches 165°F (74°C), about 8–12 minutes depending on piece size.
Remove the chicken with tongs or a slotted spoon and drain on a plate lined with paper towels.
Serve immediately with Thai sweet chili sauce if desired.