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.

Note: Sherry is a type of fortified wine originally from southern Spain. Don't use the "cooking sherry" sold in many supermarkets - it can be surprisingly high in sodium. Instead, purchase dry sherry that's sold with other fortified wines in your wine or liquor store.

Tip: You'll need about 1 1/2 pounds untrimmed boneless, skinless chicken thighs to serve four people. For recipes that call for one large thigh per person, buy them at the butcher counter; prepackaged thighs vary dramatically in size. Ask for four 6-ounce boneless, skinless thighs. To trim them well, we like to use kitchen shears to snip the fat away from the meat. After trimming, you'll have four 4-ounce portions.

Tip: To oil the grill rack: Oil a folded paper towel, hold it with tongs and rub it over the rack. (Do not use cooking spray on a hot grill.) When grilling delicate foods like tofu and fish, it is helpful to spray the food with cooking spray.