For the dressing: In a small bowl, whisk together the lime juice, fish sauce, brown sugar, rice powder and chile powder until combined. Set aside.
For the larb: In a large bowl, combine the chicken, scallions, onion, cilantro and mint, then slowly pour the dressing over the top while tossing. Mix gently but thoroughly. Taste and adjust any seasonings as needed. The larb should be tangy, salty and a little spicy (like me).