In a small saucepan, combine soy sauce, granulated sugar, brown sugar, garlic, ginger, black pepper, cinnamon stick, and pineapple juice. Bring the mixture to a boil over high heat, then reduce to low and stir until the sugars dissolve, approximately 3 minutes. Remove from heat, allow to cool slightly, discard the cinnamon stick, and incorporate ½ cup water.

Transfer the chicken thighs into a large resealable plastic bag and pour the cooled soy mixture over them. Seal the bag and ensure the chicken is well coated. Refrigerate for at least 1 hour or overnight for deeper flavor infusion.

Extract the marinated chicken and reserve the liquid. Place the marinade in a small saucepan and bring it to a boil over high heat. Reduce to a low simmer, then whisk 2 tablespoons water into the cornstarch and stir this slurry into the pan. Continue stirring until the sauce thickens slightly, then gradually blend in about ½ cup additional water to achieve a consistency similar to heavy cream. Remove from heat.

Preheat a grill or broiler. Lightly coat the chicken with the prepared sauce and cook for roughly 3 minutes on each side. Concurrently, bring the reserved sauce back to a boil, then reduce to a gentle simmer, thinning with water as necessary to maintain a pourable glaze. Slice the grilled chicken into strips, arrange on serving plates, and generously spoon over the warmed sauce.