Prepare the marinade: Place the chicken thighs in a large bowl or plastic bag. In a bowl, mix the brown sugar with the soy sauce and pineapple juice until almost all of the sugar has dissolved.
Add the garlic, ginger and onion; mix well. Add to the chicken and let marinate in the refrigerator for at least 4 hours.
Grill the chicken: Place a 13x9" baking dish on the grill or in the oven (on "Warm") if you don't have room. Add the butter and let it melt. Let the chicken come to room temperature for about 30 minutes.
Grill the chicken over indirect heat for about 30-45 minutes or until done. Remove the grilled chicken from the grill and place in the melted butter, turning each piece to coat completely.