In a medium mixing bowl, whisk together the brown sugar, pineapple juice, ketchup, soy sauce, sherry, ginger and garlic.
Use half of the sauce for the chicken marinade.
Place the other half of the sauce in a small saucepan and bring to a low simmer.
In a separate bowl, whisk together the cornstarch with 1/4 cup water. Whisk into the sauce and continue heat and whisk until it thickens and start to become glossy.
Set it aside for serving and dipping.
For the Chicken:
Combine the raw chicken with half of the huli huli sauce in a large bowl, tossing to coat. Cover and marinade in the refrigerator for at least 4 hours, but up to 1 day.
Remove the chicken and discard excess marinade. Heat the grill to medium high heat (about 350°F) and oil the grates.
Grill the chicken for 5-6 minutes on each side or until they reach an internal temperature of 165°F using an instant read digital thermometer. Remove and allow to rest while you grill the pineapple rings.
Place the pineapple rings directly on the grates for 1-2 minutes or until they get grill marks.
Remove and serve the chicken with the pineapple rings, garnish with chopped scallions and a side of the thickened huli huli sauce. If desired, you can baste with the cooked huli huli sauce before serving instead of serving on the side.