In large bowl, combine the fish sauce and chicken. and gently toss the chicken in to get it coated evenly. Marinate the chicken for minimum 20 minutes (or up to 2 hours)so the chicken can soak up all the flavor.
Gently stir up the chicken in the marinade once so that all the pieces of chicken get an even soak in the marinade.
Coat the chicken: In a large bowl, combine the potato starch
Heat a large frying pan or Dutch oven. Fill the pan with oil about 1-inch deep. Heat the oil to 350-375°F.
Gently remove the chicken from the marinade and let the excess drip off. Dip the chicken in to the Potator Starch. Allow excess to drip off and place the chicken in the hot oil.
fry the chicken on both sides until crispy and cooked through, about 25-30 minutes. IF doubling the recipe, I recommend doing it in 2 batches so you don't overcrowd the pan and undercook chicken.
While chicken is frying, combine all the nuoc cham ingredients and set aside.
Prepare the scallion oil: Heat 2 Tbsp oil in a pan and add the scallions. Cook on low heat for 3-5 minutes.
Once the chicken is done, let it cool on a cooling rack or paper towels for 5-10 minutes. Cut chicken into pieces or serve as is. Pour the scallion oil over and serve on a bed of white rice with the dressing.