Whisk sherry, soy sauce, brown sugar and the 1/3 cup pineapple juice in a large bowl. Add pineapple rings and chicken and gently stir to coat. Refrigerate for 15 minutes.
Meanwhile, preheat grill to medium-high.
Remove the chicken and pineapple from the marinade and pat dry; reserve the marinade. Oil the grill rack. Grill the chicken and pineapple until the chicken is cooked through and the pineapple is marked, 4 to 5 minutes per side.
Whisk the reserved marinade and cornstarch in a small saucepan. Bring to a boil and cook, whisking, until reduced and thickened, 2 to 4 minutes. Stir in butter. Serve the chicken and pineapple drizzled with the sauce.
Serve with brown rice and snow peas.