1. Marinade the chicken with fish sauce and brown sugar for about 1/2 hr. In the meantime in a small bowl make the sauce by combining 1 tablespoon brown sugar, 1/3 cup water, rice vinegar, and fish sauce and mix until dissolve. Add the lemongrass, shallots, and garlic and set aside.
2. Heat a large pan on high with cooking oil and add the chicken with the flat surface of thigh facing down. Allow to sear without touching them for a minute or so and then pour in about half of the sauce. Try not to move around the chicken as you want the chicken to sear and caramelize.
3. The sauce will start to thicken and the chicken will brown after a few minutes. Check to see if it’s nicely caramelized, then turn the pieces of chicken over and pour in the rest of the sauce. Continue to cook until sauce is reduced to nice thick consistency and just as you turn off the heat, toss in the basil and green onions.
4. Transfer to serving platter and generously top with roasted sesame and fresh cracked pepper.