Place chicken in a bowl with garlic, coriander root, pepper, fish sauce, 2 teaspoons of caster sugar, chilli powder and 1 tsp. salt and mix to coat chicken.
Cover and refrigerate for 2 hours.
Place rice flour in a shallow bowl. Coat chicken pieces in flour, shaking to remove excess.
Fill a large saucepan or deep-fryer one-third full with oil and heat over medium heat to 180C (or until a cube of bread turns golden in 10 seconds).
Working in batches, place chicken in oil and cook for 3 minutes or until golden. Remove using a slotted spoon and drain on paper towel.
Place remaining 60 ml of oil in a wok or saucepan over medium heat, add nahm prik pao and remaining 2 teaspoons of sugar and stir to combine and dissolve sugar.
Add chicken and cook, stirring gently, for 5 minutes or until chicken is cooked through and lightly caramelised.
Serve with rice, coriander sprigs and chilli sauce.